Answer: B
Step-by-step explanation: You have to make sure the two small sides add up to be greater than the biggest side( in this case 15in)
12+9=21, 21>15 therefore 9 would work!
I eliminated C and D using only visual clues
why 27 is a no-go: if we did it the other way around where 27 is the biggest number, it would be 12+15=27, 27=27 *therefore since 27 is equal to 27 the three sides would not make a triangle*
why 19 is a no-go: if we did this the other way around where 19 is the biggest number, it would be 12+15=27, 27>19. *visual part* Although this seems like it would also work, does <AX really look like it's bigger than <WX or <AW XD