The correct answer is the first one: "The memorial designed by Henry Bacon is the Lincoln Memorial."
The second answer ("The memorial, designed by Henry Bacon, is the Lincoln Memorial.") cannot be possible because the commas cause that the information provided between them ("designed by Henry Bacon") becomes an apposition to the main subject of the sentence. An apposition adds extra information on the nucleus of the subject of the sentence, extra information that becomes irrelevant and thus, possible to drop it, leaving a reiterative, lack of meaning sentence behind: "The memorial [dropped apposition] is the Lincoln Memorial".
The third answer ("The memorial designed by Henry Bacon, is the Lincoln Memorial.") is also wrong because the comma before the main verb separates subject from predicate and, thus, the sentence is ungrammatical.
So, in conclusion, the correct answer is: "The memorial designed by Henry Bacon is the Lincoln Memorial."
