Final answer:
Without specific pricing details from Healthy Gym, it is not possible to determine the correct cost equation for the number of days at the gym. Typically, gym costs involve a flat fee and/or a per-day rate, represented by a constant and a coefficient in front of the variable respectively. option D is the correct answer.
Step-by-step explanation:
The question posed does not provide sufficient context or information to determine the correct equation for the cost C for D days at Healthy Gym. Additional details about the gym's pricing structure are necessary to ascertain the correct formula. However, typical gym cost structures involve either a flat fee, a per-day fee, or a combination of both. Based on the given answer options, it appears that the question implies a linear relationship between the number of days and the total cost, likely involving a base membership fee and a per-day rate.
One might conjecture that the base fee is given by the constant in the equation, while the variable cost per day is represented by the coefficient in front of D. To provide an accurate answer, the specific cost details from Healthy Gym must be known.