Thanks for the post, Abhijit!

This question has to do with logical comparisons. In other words, it should be logical to compare the two things that are being compared in a sentence. For example:

Correct: Sally's hair is longer than Penny's (hair).
Incorrect: Sally's hair is longer than Penny. (Unless Sally's hair is longer than Penny is tall!)

Unlike that of earlier works on slavery, Blassingame's innovative study relies not on the records of White slave owners but on the records of the slaves themselves, especially the 70 or so autobiographies and memoirs that have been preserved.

A. that of earlier words on slavery, Blassingame's
B. that of earlier works on slavery, Blassingame in his
C. earlier works on slavery, Blassingame in his
D. earlier works on slavery, Blassingame's
E. the earlier works on slavery, Blassingame in his

(A) Incorrect. This sentence compares the singular that with Blassingame's study using the construction Unlike A... B. In other words, we have: Unlike [the study] of earlier works on slavery, Blassingame's study... which does not make sense given that the difference between the studies is one of the source of the material (slaves or slave "owners"), not when the source materials were written (earlier or later).
(B) Incorrect. Here, that is compared with Blassingame -- a who!
(C) Incorrect. This sentence compares earlier works with Blassingame.
(D) Correct. This sentence compares earlier works with Blassingame's study. This comparison makes sense.
(E) Incorrect. This choice compares earlier works with Blassingame.

Again, always check the two things being compared in a sentence to make sure the comparison is sensible!

Keep up the good work!