How am i supposed to find out which one is smallest?

Rule: For any triangle, the smallest angle is always opposite the smallest side. 
(a similar rule is that the largest angle is opposite the largest side)

We see that AC = 9 is the shortest side. Angle B is opposite this side. Therefore, angle B is the smallest angle. We don't know what angle B is in terms of a number and we don't need to find out. 

choice B) angle B

side note: we could use the law of cosines to actually compute the numeric value of angle B, but that's more work than needed