API Std 936 is very specific about the consequences of failing workability index tests:
"Plastic refractory material that fails to meet the minimum manufacturer's workability index shall be subject to requalification testing or discarded."
- API Std 936, Section 6.4.6
No provision is made for adding water or extending mixing, as this would compromise material performance.
Thus:
A is correct.
B and C violate the standard.
D is not a prescribed corrective action in this case.

This definition is rooted in petroleum refining terminology used in the context of fluid catalytic cracking (FCC) units:
"Reactor: The vessel in which the cracking reaction occurs or is completed and in which the product gases are separated from coke and/or catalyst particulate."
- API Std 936, Definitions, Section 3
Option B is the correct term describing this component. A stripper removes volatiles, a regenerator burns off coke from spent catalyst, and a standpipe is used for catalyst transport-not for the cracking reaction itself.