Step 1: Understanding the Question:
This question tests knowledge of thinning operations and selection criteria.
Step 2: Detailed Explanation:
The first trees to be removed in a thinning operation are diseased trees.
This is followed by malformed, suppressed, and less vigorous trees.
Underdeveloped trees (A): Removed later.
Fruitless trees (C): Not a primary selection criterion.
Healthy trees (D): Never removed.
Thus, the correct answer is diseased trees.
Step 3: Final Answer:
Thus, diseased trees are the first to be removed in a thinning operation, which corresponds to option (B).
[0.5cm]