Rule that allows financial institutions to calculate simple interest using 360 days in a year is called

The correct answer is: Banker's rule

Explanation:
According to Banker's rule, we take EXACT number of days to find the simple interest. For example, let's say we have the principle amount $100 with 5% interest rate, we use the following formula to find the simple interest using Banker's rule:

SI = P * I * (t/360)

Where SI = Simple interest
P = Principle amount = $100
t = Exact days = 360 (in this case)
I = interest = 5% = 0.05

SI = $5 (after plugging in the values)

Hence total amount = $100 + $5 = $105 (after 360 days)

The correct answer is Banker's rule!