Roberta is irritable only when she is tired, and loses things only

If A, then B.
A = SUFFICIENT condition
B = NECESSARY condition

If John is in Times Square, then John is in New York.
Here:
John's presence in Times Square is SUFFICIENT to conclude that he is in New York.
For John to be in Times Square, it is NECESSARY that he be in New York.


Premise:
Roberta loses things only when she is tired.
Rephrased as an if-then statement:
If Roberta loses things, then Roberta is tired.
Here, the blue portion is SUFFICIENT to conclude that Roberta is tired.

Additional premise:
Roberta has just lost her keys.
Here, the loss of the keys is an example of the blue portion above and thus is sufficient to conclude the Roberta is tired.

Option A:
Infers from a correlation between tiredness and yawning that tiredness causes yawning.
Since the loss of the keys is sufficient to conclude that Roberta is tired, option A does not describe a flaw in the argument.
Eliminate A.

Premise:
Roberta is irritable only when she is tired.
Rephrased as an if-then statement:
If Roberta is irritable, then Roberta is tired.
Here:
Roberta is tired = NECESSARY condition

End of the argument:
Since she has just lost her keys -- and thus must be tired, as shown above -- she is almost certainly irritable.
Rephrased as an if-then statement:
If Roberta has just lost her keys -- implying that she is tired -- then Roberta is almost certainly irritable.
Here:
Roberta is tired = SUFFICIENT condition

Whereas the premise considers tiredness a NECESSARY condition, the end of the argument unjustifiably deems it a SUFFICIENT condition.
Option E describes this flaw:
The reasoning is flawed in that it takes a necessary condition...to be a sufficient condition.