Final answer:
The first concern of a caterer is the D. budget of an event, as it impacts all aspects of event planning from food to guest accommodations.
Step-by-step explanation:
The first concern of a caterer is the budget of an event. When planning an event, caterers must first establish the budget as it determines the scope of the event, including the quality and quantity of food, the number of guests that can be accommodated, and the overall presentation and theme. A clear budget allows the caterer to allocate resources efficiently and make necessary adjustments to meet the financial constraints while striving to deliver the best possible experience for the attendees.