The correct option is C I hoped that it was true.
A Complex Sentence consists of one principal clause and any number of subordinate clauses.
A Noun Clause is a subordinate clause which does the work of a noun in a complex sentence.
In this case, Option C is correct. The subordinate clause that it was true acts as the object of the transitive verb hoped in the principal clause I hoped. The object is always either a noun or a pronoun. Hence, 'that it was true' is a subordinate noun clause.
Option A is incorrect because the sentences have been joined by using an adverb clause.
Option B is incorrect because it's a simple sentence and not a complex one.
Option D is incorrect because the sentences have been joined by using an adjective clause.