I need help with GRE RC assumption questions.

Let’s back up to the beginning.

Every argument has premises and a conclusion or conclusions. Premises are statements that are held to be true. Generally, these are explicitly stated facts that are held to be true — they're not debatable.

Premises are part of what supports an argument's conclusion. Assumptions are something else that supports the conclusion, but they are not stated in the argument.

Assumptions are the unstated parts of an argument that must be true in order for the conclusion to be true. 

Assumption question ask us to identify these hidden parts of the argument. To do so, we need to identify the argument's constituent parts.

Consider the following example argument:

“Jen runs an hour a day, five days a week. Therefore, she’ll take first place in the Boston Marathon.”

The premise is that Jen runs a certain amount. The conclusion is that she’ll be ready for a marathon.

Now, let’s think about this. Is it a certainty that Jen will win the Boston Marathon (conclusion) by running for an hour a day five days a week (premise)? No. There’s an unstated assumption here: the amount Jen runs guarantees that she'll be the fastest runner in the Boston Marathon. The conclusion will only follow from the premise if this assumption is true. (Of course, this assumption is almost certainly false, and therefore this argument is flawed.)

Assumption questions on the GRE are obviously more complicated than our example. But the basic approach remains the same:

1. Identify the premises and conclusion of the paragraph argument. Some things to keep in mind:

• Premises are statements that are held to be true — they are usually facts, and are usually presented at the beginning of the argument.
• Identifying the conclusion first helps point out the premises of the argument.

2. Ask yourself, “What’s missing here? What needs to be true for the conclusion to follow from the premises?” That's the assumption.
3. Evaluate the answer options according to the assumption identified.

If you are curious about more strategies concerning assumption questions, Chris wrote an helpful article that you can check out here: “GRE Critical Reasoning Question Type: Assumption Question.” Finally, a very useful strategy for evaluating whether an answer choice is indeed an assumption is called "Negating the Assumption." I highly recommend learning this strategy, which Mike covers in a GMAT post here.