Explanation:

Class B networks are defined as networks in which the first two octets of the IP address are used for network address and the last two octets are used for host addresses. The default subnet mask for a Class B network is 255.255.0.0. This means that the network portion of the IP address is represented by the first two octets (16 bits) and the host portion is represented by the last two octets (16 bits).

Subnetting:

Subnetting is the process of dividing a large network into smaller subnetworks. This can be done by borrowing bits from the host portion of the IP address and using them to create subnets. The subnet mask is used to identify the network portion and the host portion of the IP address.

For example, if we want to divide a Class B network into smaller subnetworks, we can borrow some bits from the host portion of the IP address to create subnets. The subnet mask will then be changed to reflect the new number of bits used for the network portion.

Other options:

a) 255.255.255.0: This is the default subnet mask for a Class C network. It is not the default subnet mask for a Class B network.

c) 255.255.192.0: This subnet mask allows for 6 bits to be used for subnets and 10 bits to be used for hosts. This is not the default subnet mask for a Class B network.

d) 255.0.0.0: This is the default subnet mask for a Class A network. It is not the default subnet mask for a Class B network.