Sam paid $4.50 to have two shirts cleaned.
If the dry cleaning service charges the same amount for any shirt then it means that for each shirt the rate is

If there are n shirts then the total cost (c) of the shirts can be represented by the following equation.

If you substitute n = 2 shirts into the above equation then the total cost will be $4.50 which satisfies the given information.
Therefore, the correct answer is option B