Roberta is irritable only when she is tired, and loses things only when she is tired.
Conclusion
Roberta is almost certainly irritable.
Evidence (since)
Roberta has been yawning all day and has lost her keys.
She is irritable only when she is tired, and loses things only when she is tired.
Evaluate
Whenever we're doing Flaw and see any conditional logic in the premises, we immediately suspect that we're headed into the #1 most famous flaw, Necessary vs. Sufficient.
Here, the conditional logic is signified by "only when", which is the same as "only if", a necessary indicator (right side of the arrow). So here are the two rules:
Irritable → Tired Loses Things → Tired
Since Roberta has lost her keys, we know from the 2nd rule that she is definitely tired. The author mentions that she's been yawning all day, but we don't actually need that idea at all as a "premise" (we don't have to worry whether all-day yawning = tired, because the 2nd rule tells us that if she lost her keys, then she's tired).
The author is then moving from proving that Roberta is tired to concluding that she must be irritable. That, however, is an illegal reversal. The author is arguing like this:
if she's Tired, then she must be Irritable
But the conditional goes the other way: Irritable → Tired
Goal
Look for an answer complaining about the illegal reversal (aka 'confusing necessary with sufficient'). The author does soften the conclusion by saying almost-certainly, so the answer may reflect that. But the argument is failing to consider that Roberta might frequently be tired but not irritable.