Creating applications in ServiceNow can help businesses replace outdated, inadequate, custom business applications and processes, extend service delivery and management to all enterprise departments, and extend the value of ServiceNow by leveraging its platform capabilities and integrations1234.
Reference = 1: What are Application Services? - ServiceNow 2: What is Application Development? - ServiceNow 3: Custom Applications in ServiceNow - The Cloud People 4: Build Custom Apps in ServiceNow - eBook - ServiceNow

https://developer.servicenow.com/dev.do#!/learn/learning-plans/quebec/servicenow_administrator/app_store_lea The objects that you can use in a Scheduled Script Execution (Scheduled Job) script are GlideSystem and GlideRecord. GlideSystem provides methods for performing system operations, such as logging, running background scripts, or getting system information. GlideRecord provides methods for working with records in the database, such as querying, updating, inserting, or deleting records. The current object is not available in Scheduled Script Execution scripts, as it refers to the current record on a form or list. The GlideUser object is also not available, as it refers to the current user session. Reference: Scheduled Script Execution, GlideSystem, GlideRecord

https://www.servicenow.com/community/it-service-management-forum/email-notification/m-p/695221 Reference:
When configuring the content of an Email Notification, the following syntax should be used to reference the properties of an event triggering the Notification:
event.<propertyname>.ThisisthecorrectsyntaxtoaccessthepropertiesoftheeventrecordthattriggeredtheEmailNotification,suchasevent.name,event.parm1,orevent.parm2.Forexample,{event.parm1} will display the value of the first parameter of the event.
The following syntaxes are not correct for referencing the properties of an event triggering the Notification:
current.<propertyname>.Thisisthesyntaxtoaccessthepropertiesofthecurrentrecordthatisassociatedwiththeevent,suchascurrent.number,current.shortd​escription,orcurrent.state.Forexample,{current.short_description} will display the short description of the current record.
${property name>.getDisplayValue()}. This is the syntax to access the display value of a property of the current record, such as current.state.getDisplayValue(), current.assigned_to.getDisplayValue(), or current.category.getDisplayValue(). For example, current.state.getDisplayValue() will display the state of the current record in a human-readable format, such as New, In Progress, or Closed.
${gs.<property name>}. This is the syntax to access the properties of the GlideSystem (gs) object, which provides methods for performing system operations, such as gs.now(), gs.getUserID(), or gs.getProperty(). For example, gs.now() will display the current date and time of the system. Reference: Email Notifications, Email Notification Variables

One of the uses of the ServiceNow REST API Explorer is to create sample code for sending REST requests to ServiceNow. The REST API Explorer is a tool that allows you to discover and test the ServiceNow REST APIs. You can select an API endpoint, set the HTTP method, parameters, headers, and body, and then execute the request. The REST API Explorer will show you the response status, headers, and body, as well as generate sample code for various languages and frameworks, such as cURL, Java, JavaScript, Node.js, Python, Ruby, and more. References: [Use the REST API Explorer - Product Documentation: Tokyo - ServiceNow], [Introduction to Scripted REST APIs - ServiceNow Developers]

Comprehensive and Detailed Step-by-Step
Reference Field Type:
The Reference field type is used to create a relationship between two tables. It allows you to select and display records from another table directly on a form.
For example, adding a "Caller" field to an Incident form enables you to query and select records from the sys_user table.
Why Other Options Are Incorrect:
Option B (Phone Number field type): This is used for storing phone numbers, not for querying records from another table.
Option C (String field type): While this stores plain text, it does not allow querying or establishing relationships with other tables.
Option D (Date field type): This is used for date or time values, not for referencing another table.
Reference:
ServiceNow Developer Documentation on Field Types.