Final answer:
Soup is typically the standard substitution for a free dinner salad in a restaurant. Appetizers, desserts, and breadsticks are generally not considered equivalent replacements for a salad course.
Step-by-step explanation:
The other options provided, such as appetizer, dessert, and breadsticks, do not typically classify under standard substitutions for a salad in the context of dining options. In most restaurants, soup is the common alternative offered to a dinner salad as part of a meal combo or a special promotion. Both salad and soup are considered starters or the first course of a meal, making it a more akin substitution.
Whereas, appetizers can consist of a wider variety of foods and desserts are usually served after the main course. Breadsticks might occasionally be exchanged for a salad, but they are generally categorized as a complementary side rather than a direct substitute.