MAC Address
A Media Access Control (MAC) address is a unique identifier assigned to a network interface controller (NIC) for communications at the data link layer of a network segment. It is a hardware address that consists of a series of alphanumeric characters. MAC addresses are used to uniquely identify devices on a network.

Length of MAC Address
The length of a MAC address is determined by the number of bits it contains. Each bit can be either a 0 or a 1, and the number of bits determines the total number of possible combinations and hence the number of unique addresses.

Explanation of Options
a) 24 bits: This option is incorrect because a 24-bit MAC address would provide only 2^24 (16,777,216) unique combinations. This is not enough to uniquely identify devices on a large-scale network.

b) 36 bits: This option is incorrect because a 36-bit MAC address would provide only 2^36 (68,719,476,736) unique combinations. While this is a larger number compared to a 24-bit address, it is still not sufficient for modern networks with a large number of devices.

c) 42 bits: This option is incorrect because a 42-bit MAC address would provide only 2^42 (4,398,046,511,104) unique combinations. Although this is a larger number than the previous options, it is still not enough for the scale of networks today.

d) 48 bits: This option is correct because a 48-bit MAC address provides 2^48 (281,474,976,710,656) unique combinations. This is currently the most common length for MAC addresses and allows for a large number of devices to be uniquely identified on a network.

Conclusion
The correct answer is option 'D', which states that a MAC address is 48 bits long. This length provides a large enough address space to uniquely identify devices on a network.