Step 1: Understanding the Question:
This question tests knowledge of experimental designs in forestry.
Step 2: Detailed Explanation:
RCBD is not suitable for spacing cum irrigation trials because such trials involve interactions between factors.
RCBD is suitable for single-factor experiments.
Spacing trial (A): Suitable.
Provenance trial (B): Suitable.
Nursery trials (D): Suitable.
Thus, spacing cum irrigation trials are not suitable for RCBD.
Step 3: Final Answer:
Thus, RCBD is NOT suitable for spacing cum irrigation trials, which corresponds to option (C).
[0.5cm]