52.

Answer and Explanation

Your Answer is

Correct Answer is J

Explanation

The attributive clause that/which can be omitted as the object in the clause. The sentence here should be "its presence a mystery that/which we didn't care to solve" is omitted, item J is correct.

Item F/H: solve is a transitive verb, which needs to be followed by an object, so the underlined part cannot form an independent sentence.