Final answer:
The opposite of 'insidious' is 'direct', as it implies straightforwardness and clear intentions as opposed to hidden and harmful effects.
Step-by-step explanation:
The word insidious typically describes something that proceeds in a subtle way but with harmful effects. It often refers to things that are not what they seem on the surface and can sneak up on you. The correct opposite of insidious is (b) direct. When something is direct, it is straightforward and clear, without any hidden intentions or deceptions. Unlike the insidious, which may have a 'flattering and often brilliant exterior', as mentioned in the reference, a direct approach doesn't deceive or mislead. It is explicit, evident and without need of interpretation, in contrast to the implicit nature of the insidious.