A contextual-meaning question asks which sense the sentence uses, not which sense the word most often has.

Every option in these questions is a real meaning of the word. What makes three of them wrong is the sentence, so the sentence is what you read — the dictionary is not being tested.

The bank of the river had been reinforced. — 'bank' means the edge of a river.

the sentence supplies 'river'

The bank of the river had been reinforced. — 'bank' means a financial institution.

the commonest sense, and the wrong one here

Where it bites

When the commonest sense is offered first. The exam relies on a candidate recognising the word and stopping there.