Pick the most concise way of saying something.
Every time something this weird happens, we suspect it had been one of Danny’s pranks.
a. NO CHANGE
b. have been
c. were
d. must be
Source: Barron’s ACT, 2nd Edition
D is correct because as the introductory clause (every time…happens) indicates, the idea is of an ongoing condition. The present tense is necessary. Must be is the only choice in the present tense. A simple is would also be correct, but that is not one of the choices.