The correct option is A True
A that-clause, when used as a noun clause, performs the function of a noun, as the subject or object of a verb in the sentence. It can also combine two sentences as one.
For example, in the given sentence, "that you have a foolproof plan" is a noun clause that functions as the subject of the verb "is known". It also combines the latter pair of sentences: "You have a foolproof plan. It is known to all of us." ('It' is a pronoun that refers to the previous sentence, which when combined, is no longer required.)
Hence the given statement is true and option A is correct, B is incorrect.