This question is from one of the practice tests in the Shikanzen Master N2 Grammar.
あの人が( ) 。遊んでばかりですよ。
a) まじめなことですか
b) まじめなものですか
c) まじめなはずですか
d) まじめというものですか
I answered c as my thought process was that it says something along the lines of: "Was that guy supposed to be serious? He is playing around all day!". However, the correct answer is b. In the first place, I don't understand what the meaning of the sentence is. Additionally, to solve this question, the key seems to be the difference between もの and こと, but since the meaning is not crystal clear to me, I went for the other option. I was wondering why b is the correct answer and what the sentence actually means.