Answer:
Linear decreasing function best model this situation and the required function is
f(x)=4x+18
Explanation:
It is given that a ski resort has 18 inches of snow on the ground. The snow is falling at a rate of 4 inches per hour. 
If a function has constant rate of change, then the it is a linear function.
It the given case the rate of change is constant so linear function best model this situation.
The slope intercept form of linear function is
 ... (1)
 ... (1)
where, m is slope and b is y-intercept or initial value.
Ski resort has 18 inches of snow on the ground it means initial value is 18.
The snow is falling at a rate of 4 inches per hour. So, m=4.
Substitute m=4 and b=18 in equation (1).

Therefore the required function is f(x)=4x+18.