To determine how many cups of biscuit mix are needed to make 18 biscuits, we can first find out how much mix is needed for one biscuit.
The recipe for 50 biscuits requires 9 cups of biscuit mix. Therefore, the amount of biscuit mix needed for one biscuit is:
\[ \text{Mix per biscuit} = \frac{9 \text{ cups}}{50 \text{ biscuits}} = 0.18 \text{ cups per biscuit} \]
Now, to find out how much mix is needed for 18 biscuits, we can multiply the amount needed for one biscuit by 18:
\[ \text{Mix for 18 biscuits} = 0.18 \text{ cups per biscuit} \times 18 \text{ biscuits} = 3.24 \text{ cups} \]
Since we need to choose an answer from the provided options and the closest whole number is 3 cups, the answer is:
B. 3 cups