Which sentence is grammatically correct?

The sentence establishes a past-tense context with the phrase 'who was misdiagnosed'. To maintain temporal consistency (tense harmony), the main verb must also be in the past tense ('thrived') rather than the present ('thrives') or the base form ('thrive').

Step-by-step Derivation:
Step 1: Analyze the subject. The subject is 'This child', which is a third-person singular noun.
Step 2: Analyze the relative clause. The clause 'who was misdiagnosed' uses the past tense ('was'). This sets the temporal frame of the sentence in the past.
Step 3: Evaluate Option A. 'thrive' is the base form/plural present. A third-person singular subject requires 'thrives' (present) or 'thrived' (past). This is grammatically incorrect.
Step 4: Evaluate Option B. 'thrives' is third-person singular present. While grammatically possible in isolation, it creates a tense shift from the past-tense relative clause ('was misdiagnosed') to the present, which is logically inconsistent in this specific narrative context.
Step 5: Evaluate Option C. 'thrived' is the past tense form. It agrees with the singular subject and maintains tense consistency with the relative clause 'was misdiagnosed'.
Step 6: Conclusion. Option C is the most grammatically and contextually accurate.