According to gottlob frege, "mark twain" and "samuel clemens" have
(a) the same sense but different references
(b) the same reference but different senses
(c) the same reference and the same sense
(d) different references and different senses
(e) none of these answers

According to Gottlob Frege, "Mark Twain" and "Samuel Clemens" have (B) the same reference but different senses

Explanation:

You have to consider that Mark Twain and Samuel Clemens are the same person. Then, Frege's expression refers to the fact that the words that we use in our language have a certain meaning and this meaning depends on the compression and context. One can think that the phrases "Mark Twain was a novelist" and "Samuel Clemens was a novelist" are the same since we are talking about the same person, but according to Frege these two sentences don't have the same meaning, because one person can believe one but no the other.  

So the sentence "the same reference but different senses" means that this two name refers to the same person, but the sense or meaning is different because each name can be persieve in a different way by every person.