If... what they must do. a. was not explained them
b. was not explained them.
c. was not being explained them
d. was not explained to them​

d. was not explained to them​

Объяснение:

It was not explained to them​ what they must do.

Им не объяснили, что они должны делать.

Остальные варианты не подходят. Глагол explain всегда употребляется с предлогом to (указывающим на того, кому объясняют)