Hello!
A: no referent for "that" in the sentence. It's a 100% mistake on the GMAT.
B: written as it is, it's ungrammatical. There're two verbs: "is" and "practices". Ok, "is" in general can tolerate some additional verb forms (ing-form or Past Participle: is practicing, is practiced -- from a purely grammatical point of view, no attention to meaning), but "practices" certainly can't. If we have some other verb in the clause that already contains "practices", it has to be a parallel verb: "he practices and gets better". Strict parallelism is not required; the second verb can be in another tense or with modals: "he practices and eventually is going to win", but these two verbs have to convey different ideas. Here "practices" and "is" are trying to say the same thing: that Mark practices or that Mark is practicing.
The second mistake here is tense. In the first part of the sentence the author used Present Perfect; we expect him to use the same tense in the second part as well (here it is Present Simple). We will call it parallelism. This is not a 100% mistake, because you are not obliged by grammar rules to keep the same tenses throughout the sentence (check OG13, Diagnostic test, problem 46: Present Simple "make" and "do" changes to what some call Future Simple "will gesture"). But if you have a choice between keeping the same tenses and not keeping (without any apparent reason for changing tenses), choose the option where tenses are the same.
The third reason is that reverse construction (when subject and verb switch places) here is unjustified.
C: "also" and "too" together create redundancy. It is considered a 100% mistake, but be careful: on some rare occasions it might suddenly cease to be a fatal blow to the answer choice ("while" and "at the same time" in one clause are in the correct answer in the problem from GMATPrep "a recent poll indicates that many people in the United States hold a combination").
The second problem here is tense (parallelism) - see my notes above.
D: correct
E: tense (parallelism) - see above.
