Answer:
B. The person has partly retired, but still works sometimes.
Explanation:
The definition of semi-retired is, "Having retired or withdrawn from employment or an occupation but continuing to work part-time or occasionally."
Options A. and D. both describe a person who is retired.
Option C. describes a person who works full time and so not retired or partially retired.
Option B. describes a person who is partly retired, which fits the definition of semi-retired and therefore correct.