If, . . . my father had taken the pains to explain to me that the principles of Agrippa had been entirely exploded and that . . . the powers of [Agrippa] were chimerical, . . . I should certainly have thrown Agrippa aside. . .

A chimera is a deception or fantasy of the mind.

What is the effect of using the word chimerical to describe the powers of the principles of Agrippa?

It suggests Agrippa is no more than imaginary.
It suggests this period of time was crucial to the narrator.
It suggests the principles have the illusion of truth.
It suggests the speaker is haunted by his past.

I think the answer is the third one

The correct answer is: It suggests the principles have the illusion of truth.

By describing the powers of Agrippa as chimerical, it implies that the principles may seem true or real at first, but in reality, they are deceptive or fantastical. It suggests that there is an element of illusion or false perception associated with these principles.

You are correct! The effect of using the word "chimerical" to describe the powers of the principles of Agrippa is that it suggests the principles have the illusion of truth. The word "chimerical" means something is deceptive or imaginary. By describing the powers of Agrippa as chimerical, it implies that these powers are not real or grounded in reality, but rather appear to be true or substantial. This choice of word helps convey that the principles of Agrippa are fanciful or illusory rather than genuinely powerful.