Your answer would be B.) 6 inches.
Here's the math:
The height of the photo goes from 3 in. to 18 in. To get from 3 to 18 we multiply by 6. So, we must multiply the width by 6 to make sure that our photo is up to scale after the enlargement. Well 6*1= 6, which will be your final width.
So, your answer is 6 inches wide