What is a valid multicast MAC address?

00:90:69:9f:ea:46 has a first octet of 00 (binary 00000000), where the LSB is 0, indicating a unicast MAC address.

ff:ff:ff:ff:ff:ff is the Layer 2 broadcast MAC address, not a multicast address.

Multicast MAC addresses are identified by having the least significant bit of the first octet set to 1. In the address 01:00:5e:28:12:95, the first octet is 01 (binary 00000001), where the LSB is 1, indicating it is a multicast address, and it falls within the IANA-defined range for IPv4 multicast addresses.

While the first octet 99 (binary 10011001) has its least significant bit set to 1, technically making it a multicast address, it does not fall within the commonly recognized IANA-reserved ranges for IPv4 multicast addresses, unlike option C.