The answer doesn't make sense. The question is in the past tense, but the answer is using a continuous tense. It's not correct, the answer should use the present perfect: 'His bicycle's been stolen.' Tom's bicycle's been stolen = Tom's bicycle has been stolen.

" 's " means is :) sometimes, it means "has" in past perfect structure

— What made Tom so sad? (past tense; his sadness is completed/finished/over) — His bicycle's being stolen. => His bicycle HAS been stolen. (present perfect tense)(incorrect verb tense in my opinion) => His bicycle WAS stolen. (past tense. The "stealing" action lasted only a few seconds and was then completed.) The two tenses should be consistent (past-past). Alternative: Why is Tom so sad? (present tense; ongoing sadness) His bicycle's been stolen. (has been stolen = is still missing = ongoing missing condition) (present-present perfect)

你的意思是这句话实际上是一个名词词组？

那这句话为什么用现在进行时呢，这件事是过去让Tom 不开心