Answer Description

When determining the default subnet mask of an IP you need to identify which class (A, B, C, etc.) the IP falls under. The first octet of the IP is 172 which falls in the class B range of 128 – 191. The corresponding default subnet for this range is 255.255.0.0. A network architect is not required to use the default and could design the network to use any subnet mask, but the question specifically asks for the default mask.

Wikipedia

Classless Inter-Domain Routing (CIDR ) is a method for allocating IP addresses and for IP routing The Internet Engineering Task Force introduced CIDR in 1993 to replace the previous classful network addressing architecture on the Internet Its goal was to slow the growth of routing tables on routers across the Internet, and to help slow the rapid exhaustion of IPv4 addressesIP addresses are described as consisting of two groups of bits in the address: the most significant bits are the network prefix, which identifies a whole network or subnet, and the least significant set forms the host identifier, which specifies a particular interface of a host on that network