A distributed port scan operates by:

A.
Blocking access to the scanning clients by the targeted host

B.
Using denial-of-service software against a range of TCP ports

C.
Blocking access to the targeted host by each of the distributed scanning clients

D.
Having multiple computers each scan a small number of ports, then correlating the results

Explanation:
Think of dDoS (distributed Denial of Service) where you use a large number of computers to create simultaneous traffic against a victim in order to shut them down.