The right answer is B.
The Chinese cannot be along with an end to the class system and serfdom. There is, therefore, a modification error in A.
There is a pronoun error in C. The plural pronoun they is expected to refer to the Chinese, a singular noun. C can be eliminated based on this pronoun error.
The Chinese cannot be together with the class system and serfdom. There is a modification error in D, hence this option can be eliminated.
The Chinese cannot be in addition to the class system and serfdom. This means the opening modifier in E is dangling. E can be Eliminated.
In B, it is logical for the Chinese to put an end to the class system and serfdom. Hence, there is no modification error in B compared with A, D, and E. B is the right answer.
Along with an end to the class system and serfdom, the Chinese have abolished many of the other traditional institutions of the Tobetan way of life.
(A) Along with an end to the class system and serfdom
(B) Besides putting an end to the class system and serfdom
(C) Not only did they end the class system and serfdom, but
(D) Together with the class system and serfdom coming to an end
(E) In addition to the class system and serfdom coming to an end