Jack and John __________ helpful.

A be

B have been

C was

D had been

my answer is : have been 
check the answer if not explain 

Dear student,

If you’re focusing on the present and you want to say that, up to now, “Jack and John have been helpful”, well you say that.
If you’re focusing on a time in the past and you want to say that, before that time, “Jack and John had been helpful”, then you say that.

Regards.