Step 1: Recognize “but” as contrast marker.
The sentence sets up a contrast between external appearance (unsmiling) and internal nature (likely positive).
Step 2: Choose the most contrasting idea.
Option (a) gives the contrast: though he appears unsmiling, he is compassionate. Options (b), (c), and (d) are consistent with being unsmiling, but do not offer contrast—they reinforce seriousness or strictness.
\[
\boxed{\text{(a) his heart is full of compassion for others}}
\]