Alpha rays have poor penetration ability, and can only travel a few centimeters in the air, which can be blocked by a piece of paper.

The penetration ability of beta rays is very strong, and it can easily penetrate several centimeters thick black paper and aluminum plate.

Gamma penetration ability is stronger, even through a few centimeters thick lead plate.

Because the radiation in this question can penetrate 1mm and 3mm aluminum plates, but it can't penetrate lead plates, it must contain no gamma rays, but it must contain beta rays and may also contain alpha rays.

So choose a.