<p>A little while ago, I did a study of this question using 13 past (QAS) exams (math only).</p>
<p>The results are consistent with a uniform random distribution of answers, i.e., p(A) = p(B) = p(C) = p(D) = p(E) = 0.2.</p>
<p>For a total of 572 multiple choice questions, here were the letter frequency numbers and corresponding z-scores:</p>
<p>A 108 -0.67
B 116 0.17
C 119 0.48
D 120 0.59
E 109 -0.56</p>
<p>(expected value for each: 114 to 115).</p>
<p>You would need a z-score of more than 1.96 or less than -1.96 to say with a decent level of confidence that these letter frequencies are NOT uniformly random.</p>