In the Manage phase, information about the validation of in-use features/functions can be found in the Test Plan. This document outlines the specific tests, criteria, and methods for verifying that the solution's features and functions are operating as expected.

Validation Purpose of the Test Plan: The Test Plan specifies the steps to validate that each configured feature is performing correctly and meeting the intended objectives.

Documentation of Test Results: It also includes documentation of results, which helps ensure that all features remain functional and aligned with requirements in the production environment.

Explanation of Why Other Options Are Less Likely:

Option A (Solution Infrastructure Design) and Option B (Solution Configuration Design) focus on setup and configuration rather than validation.

Option D (Business or Technical Objectives) are used for setting goals, not validating functionality.

The Test Plan is thus the correct source for information on validating in-use features/functions during the Manage phase.