Answer:
Decrease the size of the group
Step-by-step explanation:
Since in the question it is mentioned that there is a group of fifteen people for coordinating the merger of two corporations. Everyone is highly qualified but the work of a group seems to very slow.
So if Alberto wants to increase the cohesiveness and productivity, so he should decrease the group size so that the working could come in a normal cycle and every task could be completed within a prescribed time.