Step 1: Understanding mesocarp.
The mesocarp is the middle layer of the fruit. In the case of coconut, it is the fibrous part found between the outer skin (exocarp) and the seed (endocarp).
Step 2: Analyze the options.
- Hard: The outer layer, or exocarp, of coconut is hard, but the mesocarp is fibrous.
- Fibrous: The mesocarp of the coconut is known for its fibrous structure, which is used in the production of coir.
- Fleshy: The coconut does not have a fleshy mesocarp; instead, it is fibrous.
- None of these: This is incorrect because the mesocarp is indeed fibrous.
Step 3: Conclusion.
The mesocarp in coconut is fibrous. The correct answer is (B).
Final Answer:
\[
\boxed{Fibrous}
\]