21) D

This is quite a simple question actually. The argument presented in the second and third paragraphs is that newspapers can have an influence on the people reading them. In the third paragraph, it implies that since the 5 pro-leave newspapers had the most sales, there is “no surprise” that majority of UK population was “persuaded to vote Leave”. This is a classic example of causation.
Causation in the argument – since more pro-leave newspapers were sold, majority of UK population voted leave.

The weakness in this argument is this causation. It fails to consider that people may buy newspapers which express and support the views they already have.

This is why D is the answer.