Decide whether the second part is a result of the first, a similar/supporting idea, an example of it, or something that pushes back against what you might expect from the first part.

Think about what each transition typically does: Which one usually shows result, which one shows similarity, which one introduces an example, and which one introduces an unexpected or opposing point?

Read the sentence as a whole, imagining a neutral placeholder in the blank:

"Researchers ... have found that the extent of Arctic sea ice has declined dramatically since 1980; in 2012, for example, the minimum summer ice cover was roughly half the average recorded in the early 1980s. ______ some computer models still project periods of partial recovery in the coming decades."

The first part emphasizes a dramatic decline in sea ice. The second part says that some models still project periods of partial recovery in the future.

Ask: Does the second idea

• show a result of the first?
• give a similar or supporting point?
• give an example of the first?
• or say something that goes against or complicates what we would expect from the first?

Here, if ice has been shrinking dramatically, we might expect models to predict continued loss, not partial recovery. So the second clause complicates or contrasts with the first idea.

Now classify each answer choice by its usual function:

• "Therefore," signals a result or conclusion.
• "Similarly," signals a comparison or likeness.
• "For instance," introduces an example of something just mentioned.
• "Yet," introduces an idea that goes against what was just said or is surprising in that context.

Because the sentence moves from strong evidence of long-term decline to an unexpected note about possible future recovery, the transition that shows this kind of contrast—"Yet,"—is the most logical choice.