Option A is correct. Under Sub-Clause 11.9 (Performance Certificate) refusal or incorrect issuance by the Engineer, combined with disagreement by the Contractor, may cause a Dispute to arise.
Option B is correct. If a Party is dissatisfied with a determination, it must give a Notice of Dissatisfaction (NOD) within 28 days to escalate the matter to a Dispute, allowing either Party to refer it to the DAAB as per Sub-Clause 21.4.
Option C is incorrect. The contract does not specify a 42-day time limit for submission to DAAB after NOD; timelines vary by contract and stage.
Option D is incorrect. 'Disagreement' is not a formally defined term in FIDIC contracts, whereas 'Dispute' is.
References:
FIDIC Red, Yellow, Silver Books 2017 Edition, Sub-Clause 11.9 and Clause 21 - Claims, Disputes, and Adjudication FIDIC Contract Manager Study Guide, Module on Dispute Resolution

Comprehensive and Detailed Explanation:
The Employer must provide detailed notice to the Contractor regarding intended changes material to financial arrangements, enabling transparency and allowing the Contractor to assess impacts and prepare claims or adjustments accordingly.
This obligation supports fair risk allocation and project control.
References:
FIDIC Red, Yellow, Silver Books 1999 & 2017 Editions - Various clauses on Notices and Variations FIDIC Contract Manager Study Guide, Module on Communication and Financial Notices

Under the FIDIC Silver Book 1999, Clause 9 governs Tests on Completion, while Clause 12.4 addresses Tests after Completion. These provisions define performance verification before and after Taking-Over.
Option A is correct. Under Sub-Clause 9.4 [Failure to Pass Tests on Completion], if the Works fail the Tests on Completion, the Employer has several options, including accepting the Works subject to a reduction in the Contract Price. This reflects the commercial flexibility within FIDIC, allowing completion despite minor deficiencies, with financial adjustment.
Option D is also correct. Under Sub-Clause 12.4 [Failure to Pass Tests after Completion], if the Works fail post-completion performance tests, the Contractor may be required to pay performance damages. Once these damages are paid, the Works may be deemed to have satisfied the required performance levels. This mechanism ensures that operational deficiencies are compensated financially rather than requiring physical rectification in all cases.
Option B is incorrect because both Tests on Completion and Tests after Completion can coexist within the same contract; they serve different purposes (pre-taking-over vs post-taking-over performance verification).
Option C is incorrect because Tests after Completion are generally carried out by or under the responsibility of the Contractor, not solely the Employer, although the Employer may witness them.
These provisions demonstrate FIDIC's structured approach to quality assurance and risk allocation across different project stages.

Comprehensive and Detailed Explanation:
Under FIDIC Red Book 1999, advance payment is intended to assist the Contractor in financing the mobilization and early works costs before the Contractor begins receiving regular payments for work performed (Option A). It is not a payment for work already completed, so Option B is incorrect.
The General Conditions provide that the advance payment is usually made in a lump sum or agreed instalments and that the Employer makes the payment in advance (Option C is partially true but generally it can be one or multiple instalments, depending on contract terms).
Importantly, the advance payment must be repaid by the Contractor through deductions from subsequent interim payments (Option D), ensuring the Employer recovers the advanced funds as the work progresses.
References:
FIDIC Red Book 1999, Sub-Clause 14.2 - Advance Payment
FIDIC Contract Manager Study Guide, Module on Payment Procedures and Financial Management

According to FIDIC Red and Yellow Books (both 1999 and 2017 editions), the advance payment guarantee amount may be progressively reduced as the Contractor repays the advance payment through deductions from interim payment certificates. This means the guarantee is reduced as per amounts repaid, not only upon full repayment.
Option D is correct: The guarantee reduces as partial repayments are certified in Payment Certificates.
Option C is incorrect because the reduction happens gradually, not only after full repayment.
Options A and B do not directly relate to the reduction mechanism of the advance payment guarantee.
References:
FIDIC Red and Yellow Books 1999 & 2017 Editions, Sub-Clause 14.2 - Advance Payment Guarantee FIDIC Contract Manager Study Guide, Module on Payment Procedures