Concept:Pupils who offer only one subject are those in Mathematics only or English only.
Explanation:From the Venn diagram, the number of pupils who offer Mathematics only is
18.
The number of pupils who offer English only is
3.
So, pupils who offer only one subject
=18+3=21.
Pupils in the intersection,
7, offer both subjects, so they are not counted.
Answer:21 pupils offer only one subject.
Therefore, the correct option is D.
