Final answer:
The main problem encountered when maize was transplanted in the Old World was pest infestation.
Step-by-step explanation:
In Pilcher's discussion of maize in the Old World, the main problem that was encountered when maize was transplanted was C) Pest infestation. Maize was introduced to the Old World from the New World and faced challenges in adapting to new environments. One of the major difficulties was dealing with pests that attacked the maize crops, leading to decreased yields and potential crop loss.