Explanation

The writer's thesis is that foolish opinions are avoided by simple rules — chiefly, 'if the matter is one that can be settled by observation, make the observation yourself.' So through observation we can avoid many silly mistakes (B). A overstates the claim ('if they were clever' — the writer says no superhuman brain is needed), C turns one cited mistake into 'many mistakes', and D's 'all errors' contradicts 'not from all errors, but from silly errors'. Hence the answer is B.