Which of these situations best shows irony?
a. a school nurse misses work because she is not feeling well.
b. a school nurse sends five sick students home one afternoon.
c. a school nurse is married to one of the teachers.
d. a school nurse, who is usually busy, has a day with no student visits.

Option A. a school nurse misses work because she is not feeling well.

Explanation:

In literature irony is a technique to present the opposite of the expectation or "obvious action , deed or meaning".

Among all the given option, option A seems to be an irony because a school nurse,  who is responsible for taking care of health of students in the school is expected to be careful about her own health. But in contrast to the expectation, the school nurse had fallen ill herself due to which she had taken leave from work.

Thus, option A correct.

the endomembrane system i think