Use the be-passive form of the verb in brackets in the past simple. The parcels are the topic, and the packing staff are obvious: the parcels ___ before loading yesterday. (seal)

Explanation

Were sealed agrees with the plural receiver in the requested past simple be-passive and leaves the obvious packing staff unmentioned. Got sealed can be a standard informal get-passive, but it does not satisfy the instruction to use the be-passive form.