Step 1: Understanding the Question:
This question tests knowledge of tree diseases.
Step 2: Detailed Explanation:
Root-rot in Shorea robusta (sal) is caused by Ganoderma (a fungus).
It is a serious disease affecting sal forests.
Pseudomonas (A): Bacterial pathogen.
Polyporus (B): Causes heart rot.
Pythium (D): Causes damping-off.
Thus, the correct answer is Ganoderma.
Step 3: Final Answer:
Thus, Ganoderma causes root-rot in Shorea robusta, which corresponds to option (C).
[0.5cm]