C. 40 - 1.5d is the correct expression which represents the amount of money Sabina has left.
$40 is what she started with, and she paid d (or, 1d) for lunch, and then 0.5d for a snack. 1d + 0.5d = 1.5d. That's how much she spent, so you take it away from 40, therefore C is the correct option.