E for me too.
"a practice fostered by modern seed-marketing methods"
This appositional phrase is wrongly representing noun "a single strain of plant"
B. "as is" is wrong. it is qrong most of the time in GMAT
C. same as A. "a single strain of plant" is represented as "a practice"
D. still not clear
E. i will go with it. "The use..." is identified as "a practice"