Select the correct option:
So fragile __ the talks that one disclosure could sabotage them.

The correct option is were.

Explanation:
The given sentence follows an inverted sentence structure used for emphasis: "So [adjective] + [verb] + [subject]..."

1. Identify the subject:
The subject in this clause is "the talks", which is a plural noun.

2. Determine Subject-Verb Agreement:
Because the subject "the talks" is plural, it requires a plural verb. This rules out the singular verbs "is" and "was".

3. Determine the Tense:
The sentence describes an action in the past where one disclosure could sabotage them. Therefore, the past tense plural verb "were" is the grammatically correct choice to complete the sentence.

Complete Sentence:
"So fragile were the talks that one disclosure could sabotage them."