To find the total cost, in dollars, to buy \( d \) donuts when each donut costs $40.75, you can use the following equation:
\[ \text{Total Cost} = 40.75 \times d \]
So, the equation that represents the total cost is:
\[ C = 40.75d \]
where \( C \) is the total cost and \( d \) is the number of donuts.