Concept:Theoretical refers to ideas and principles; its opposite is
practical, which refers to actually doing something.
Solution:The sentence contrasts two sides of an exercise with "but": how easy it seems on paper and how hard it is to actually do.
The word opposite to "theoretical" that fits the gap is "practical":
"Some exercises may seem easy in theoretical terms but can be really difficult in practical aspects."
B. "natural" is the opposite of artificial.
C. "logical" means reasonable; it is close to theoretical, not opposite.
D. "actual" means real; it is not the standard opposite of theoretical and does not fit "actual aspects".
Key point: theory / practice and theoretical / practical are fixed opposite pairs.
Answer:Option A (practical)