答案：D

解析：

Since the daily charge for the car is $45, the tax rate is 8%, and the rental is for d days, the total rental charges can be represented by the expression 45*1.08*d.

Note that adding an 8% tax to the original amount is the same as using 108% of the original amount, which is achieved by multiplying by 1.08.

Since c(d) represents the total rental charges, including tax, over d days, the function that best models this is: c(d)=45*1.08d.