The storm frequency for a peripheral commercial area is once in 2 years (option C).

Explanation:
The storm frequency refers to the average number of storms that occur within a specific period in a particular area. It is an important parameter to consider when designing structures and infrastructure to withstand the forces imposed by storms. In this case, we are considering a peripheral commercial area, which typically includes buildings and other structures for commercial purposes.

The given options for storm frequency range from once a year to once in a month. Let's analyze each option to determine the most appropriate frequency for a peripheral commercial area.

a) Once a year: This frequency implies that a storm occurs in the area once every year. While storms can cause damage and disruption, a peripheral commercial area may be able to handle such infrequent events with proper design and maintenance. However, this frequency may still pose a significant risk, especially if the storms are severe.

b) Twice a year: This frequency suggests that storms occur in the area twice every year. This frequency increases the risk and potential damage caused by storms. It may be challenging for a peripheral commercial area to withstand frequent storms, especially if they are intense. Therefore, this option is less likely to be the correct answer.

c) Once in 2 years: This frequency implies that a storm occurs in the area once every two years. This option strikes a balance between the infrequency of storms and the need for resilience in a peripheral commercial area. With a storm frequency of once in 2 years, the area has more time to recover and prepare for the next event. This frequency is a reasonable assumption for designing structures and infrastructure in a peripheral commercial area.

d) Once in a month: This frequency suggests that a storm occurs in the area once every month. This option represents a very high storm frequency, which would pose significant challenges for a peripheral commercial area. Monthly storms could cause frequent damage, disruption, and financial losses. Therefore, this option is less likely to be the correct answer.

Considering the options given, once in 2 years (option C) is the most appropriate storm frequency for a peripheral commercial area. It strikes a balance between infrequency and the need for resilience, allowing the area to recover and prepare adequately for storms.