Which is an opinion in the passage? a) the hole measures nearly 1,000 feet across… b) one of the most breathtaking sites is the great blue hole… c) …a series of underwater tunnels might connect the hole to the mainland. d) the great blue hole is the result of repeated collapses of an underwater cave …

C the appinoin, was when she said breathtaking, blue hole something!

An opinion is a personal view; it's subjective and bias. It is not really based on fact. Note: that opinion ⇒ subjectivity fact/knowledge ⇒ objectivity

A is fact about the hole's measurement and this is objective. This isn't based on opinion or subjectivity. 
B displays an opinion, because it isn't fact that the Great Blue Hole is one of the most breathtaking sites, but rather someone's personal view. This is subjective.
C is a fact and objective. This isn't based on opinion or subjectivity. 
D is a fact about how it came to be and objective. This isn't based on opinion or subjectivity.