Since 1 person costs 50, and for each person after that the cost goes up by $5, we can conclude the following: There is a flat fee of $45 and each guest costs $5.
Since the entry fee is a flat charge and cannot change, and the amount per guest changes based on the number of guests, the variable goes after the 5.
This can be modeled as $45 + $5x. The only options that represent this are A and D. Since he wants his total to be under $100, The correct option would be d. $45 + $5x < 100. Otherwise, with answer A, he would come up with a total that goes over his limit.