The choir sang passionately, as they moved through elaborate and challenging four-part harmonies.

A) as they moved through elaborate and challenging four-part harmonies
B) as they were moving through elaborate and challenging four-part harmonies
C) moving themselves through elaborate and challenging four-part harmonies
D) as it moved through elaborate and challenging four-part harmonies
E) as it moved through challenging four-part harmonies elaborately

I marked A as correct but D is given as correct. Can someone explain to me why that is ? For example if choir marks a group of individual people , when will the verb be in singular and when in plural form ?

Give me some examples for collective nouns like the police as well ,if possible.