What is the maximum size of an IPv4 header?

The maximum size of an IPv4 header is indeed 60 bytes. This is determined by the structure of the IPv4 header itself, which consists of a 20-byte base header and an optional extension that can increase the total size.

In the base header, there are various fields including source and destination IP addresses, version, IHL (Internet Header Length), type of service, total length, identification, flags, fragment offset, time to live (TTL), protocol, header checksum, and options (if present). The IHL field specifies the length of the header in 32-bit words. Since the minimum size of an IPv4 header is 20 bytes (5 words), and the maximum is 60 bytes (15 words), this allows for an option field that can add up to 40 bytes to the standard header.

Other choices do not reflect the correct maximum size. For instance, 32 bytes would only account for a small number of options and the minimum necessary fields, while 64 bytes is beyond the defined maximum. The base header alone cannot be less than 20 bytes, so it's necessary to understand that the correct answer incorporates both the standard header and potential options allowed within the IPv4 specification.