The correct option is B We could not figure out the meaning of his words.
Complex sentence contains one dependent clause and one independent clause.
Noun Clause can be the subject or object of the verb or object of preposition.
A simple sentence contains only one independent clause, with one subject and one predicate. A complex sentence can be converted to a simple sentence reducing the dependent clause to a word or phrase.
Option B is correct: "What he said" has been converted to "meaning of his words". Hence, option B is the correct answer.
Option A and C are incorrect: The meaning of the simple sentence in these options is different from the given complex sentence.
Since option B is the correct answer, hence option D is invalid.