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.