To answer this question, the user needs to be familiar with the IEEE Ethernet designations and the different types of Ethernet cables.

Thicknet is a type of Ethernet cable that has a thicker diameter than other Ethernet cables. It is also known as 10Base5 because it can support Ethernet networks with a maximum data transfer rate of 10 Mbps over a maximum distance of 500 meters.

Now, let's go through each option and determine which one is the correct answer:

A. 10Base5: This option is correct. Thicknet is also known as 10Base5 according to IEEE Ethernet designations.

B. 10BaseT: This option is incorrect. 10BaseT is a type of Ethernet cable that uses twisted-pair wiring and can support Ethernet networks with a maximum data transfer rate of 10 Mbps over a maximum distance of 100 meters.

C. 10Base10: This option is incorrect. 10Base10 is not a valid Ethernet designation.

D. 10Base2: This option is incorrect. 10Base2 is a type of Ethernet cable that uses thin coaxial wiring and can support Ethernet networks with a maximum data transfer rate of 10 Mbps over a maximum distance of 185 meters.

Therefore, the correct answer is:

The Answer is: A. 10Base5