The opening cerenony is to be held in the lecture hall this afternoon,（）can seat 4 people.where \when\that\which

The opening cerenony is to be held in the lecture hall this afternoon,（）can seat 4 people.where \when\that\which
The opening cerenony is to be held in the lecture hall this afternoon,（）can seat 4 people.
where \when\that\which

The opening cerenony is to be held in the lecture hall this afternoon,（）can seat 4 people.where \when\that\which
不选when,是因为这个空是表示地点的,而不是表示时间.
不选that,是因为如果用that,前边不能有逗号.
不选which,除非原题改成The opening cerenony is to be held in the lecture hall this afternoon,in _____ can seat 4 people.
选where,是因为这个空刚好是表示地点的.

where 后面是“可以坐下四个人”，那当然要用表示地点的where了

选which 指先行词the lecture hall ，完整句子是the lecture hall can seat 4 people

which
seat
有...座位,容纳...人[W]
The auditorium seats 3,000 people.
这个礼堂可坐三千人。