To determine how many collectible dolls Mariah can buy, you can set up the equation based on the total amount of money she has and the cost of each doll.
Let \( d \) be the number of dolls. The cost of \( d \) dolls would be \( 4.5d \), which should equal her total amount of money, $75.00.
The correct equation representing this situation is:
\[ 4.5d = 75 \]
So, the answer is:
4.5d = 75 (4.5 d is equal to 75)