Select the correct option:

Only after ten years of fieldwork __ she truly understand the dialect’s nuances.

The correct option is did she.

Detailed Explanation:

In English grammar, when a sentence begins with a negative or restrictive adverbial expression (such as "Only after", "Never", "Hardly", "Seldom", or "Rarely"), standard word order is inverted. This rule is known as negative/restrictive inversion.

1. Inversion Rule: When an inverted structure is required, the auxiliary verb or helping verb precedes the subject, following the structure: Auxiliary Verb + Subject + Main Verb.

2. Simple Past Tense Structure: The sentence describes a completed past action ("understand"). The auxiliary verb used for simple past tense inversion is did. Therefore, the inverted order is "did" (auxiliary verb) + "she" (subject) + "understand" (base form of main verb).

3. Option Analysis:

• she had - Incorrect because it uses standard subject-verb word order without inversion.
• did she - Correct because it applies the proper inversion (Auxiliary + Subject) required after "Only after...".
• was she - Incorrect because "was" cannot be paired with the base verb "understand" in this context.
• she did - Incorrect because it does not invert the subject and auxiliary verb.

Thus, the complete correct sentence is: "Only after ten years of fieldwork did she truly understand the dialect’s nuances."