Could you expalin to me why 'a' is correct?

[color=darkblue]Psycho was … for me and at one point I almost screamed out loud in the cinema.

[color=green]a.too scary
[color=red]b.too much scary

(b) is incorrect because “too much” cannot be followed by an adjective. It can be followed by a noun, as in “too much sugar”, “too much noise”, etc.