Because UDP is a connectionless protocol: (Select 2)

A.
UDP recvfrom() and write() scanning will yield reliable results

B.
It can only be used for Connect scans

C.
It can only be used for SYN scans

D.
There is no guarantee that the UDP packets will arrive at their destination

E.
ICMP port unreachable messages may not be returned successfully

Explanation:
Neither UDP packets, nor the ICMP errors are guaranteed to arrive, so UDP scanners must also implement retransmission of packets that appear to be lost (or you will get a bunch of false positives).