Answer: To shrivel and harden
Explanations:
To get plump and juicy: First off, raisins are not plump and juicy. Also, the lines say "dry up like a raisin". This clearly lets us know that raisins are not plump and juicy.
To expand: Some things expand in the sun, but dry up doesn't make sense for exanding in the sun.
To disappear: I'm pretty sure that raisins aren't going to just dissapear in the sun -v-
To shrivel and harden: Yes. The phrase "dry up like a raisin in the sun" clearly means that it's going to shrivel. Besides, there are other things that shrivel up from the sun heat.
Anyways, here's a screenshot of raisins. :>