Can any expert please tell me why option C is correct and D wrong?

I think option C is conspicuously wrong because "Does bamboo have higher tensile strength than mild steel" is more like a question.
D, in my opinion, is error-free and clear.

Also, can "Does bamboo have higher tensile strength than mild steel" be called a substantive clause? As per this article, the substantive clause typically begins with a relative pronoun or relative adverb.
Will it be wrong to construct a sentence like this:
Does bamboo have higher tensile strength than mild steel is not yet known.