This problem is quite confusing indeed. Perhaps there must have been a typographical error in the given problem. However I dug up on other sources and I found out that the right form of the equation is:
 5 z + 16 = 34 – z
 We can see that in the box it is the minus sign which is lacking. Anyway there are only 2 options that could fill in the box, either plus or minus. If for instance we had multiplication or division in the box, it would result in more than 1 solution.
  
 So now we can solve this problem by simple operations:
 5 z + z = 34 – 16
 6 z = 18
 z = 18 / 6
 z = 3
  
 Therefore the answer is:
 G. 3