Step 1: Read the question.
We must pick an example of a cognitive skill.
Step 2: Recall the meaning.
Cognitive skills involve thinking, reasoning, and problem-solving.
Step 3: Test option A (Emotional control).
This belongs to the affective domain of feelings, so it is not cognitive.
Step 4: Test option C (Time management).
This is an executive or organisational skill, not purely cognitive.
Step 5: Test option D (Physical health).
This relates to the body and psychomotor side, so it is not cognitive.
Step 6: Confirm option B (Problem-solving ability).
Problem-solving directly uses thinking and reasoning, squarely in the cognitive domain.
Step 7: Conclude.
Hence the cognitive skill is option B.
\[ \boxed{\text{B. Problem-solving ability}} \]