Select the correct option: So delicate _______ the negotiations that a single leak could derail them. –

Explanation

The sentence begins with "So + adjective", which requires subject–verb inversion.
The subject is the negotiations, which is plural, so the correct verb is were.
Correct sentence:
"So delicate were the negotiations that a single leak could derail them."
are – Incorrect (present tense)
were – Correct (past tense, plural subject)
is – Incorrect (singular verb)
was – Incorrect (singular verb)
Correct Answer: were