In recent years cattle breeders have increasingly used crossbreeding, in part that their steers should acquire certain characteristics and partly because crossbreeding is said to provide hybrid vigor.
partly to acquire certain characteristics in their steers
Option: (B)
The sentence gives two reasons that cattle breeders use crossbreeding; these reasons should be introduced in parallel ways with the word partly. The infinitive to acquire clearly and concisely conveys the purpose of the crossbreeding.
A) In part should be partly; use of relative clause is non-grammatical.
B) Correct. In this sentence, the word partly is used to introduce both reasons; the phrase to acquire certain characteristics is clear and concise.
C) Because it suggests that crossbreeding has occurred because the steers have already acquired certain characteristics.
D) Passive voice should be acquired by is awkward and illogical.
E) This is a junk option as the action (acquiring) should be in the future form