What is the proper subnet mask for the network 192.168.55.0/27?

255.255.255.192 corresponds to a /26 subnet mask, which has 26 network bits, not 27.

255.255.255.248 corresponds to a /29 subnet mask, which has 29 network bits, not 27.

A /27 CIDR prefix indicates that the first 27 bits of the IP address are dedicated to the network portion. In the fourth octet, this translates to the first three bits being network bits (111) and the remaining five bits being host bits (00000), which in decimal is 128 + 64 + 32 = 224, making the subnet mask 255.255.255.224.

255.255.255.0 corresponds to a /24 subnet mask, which has 24 network bits, not 27.