The correct answer is 255.255.255.224. This mask corresponds to a /27 prefix, which supports up to 32 IP addresses, 30 of which are usable for hosts when accounting for the network and broadcast addresses. This fits the need for at least 28 devices. 255.255.255.0 is a /24 prefix, which supports up to 256 IP addresses and would also work but does not align with the criteria of efficient IP address usage specified in the question. 255.255.255.240 and 255.255.255.248 are /28 and /29 prefixes, supporting up to 16 and 8 IP addresses, respectively, which are insufficient for the requirement of at least 28 devices.