Final answer:
A retainer is the correct answer as it is a type of compensation based on non-contingent, professional services, differing from commission, salary, or bonuses which are dependent on sales, employment or performance, respectively.
Step-by-step explanation:
The type of compensation based on professional services rendered on a non-contingent basis is C) Retainer. A retainer fee is a prepayment for professional services that a client pays to reserve the professional's time and ensure their availability. This differs from other types of compensation like commission, which is based on the completion of a sale or transaction, salary, which is a regular payment for employment, typically paid on a monthly or biweekly basis, and bonus, which is typically a performance-based incentive paid in addition to salary or commission.