"distribution which is symmetric about mean m"
tell us that each side of m has 50% each
1. 68 % of the distribtion in the interval from m-d to m+d, inclusive

so this tells us that m+d has 68/2 % anything above m+d is 50-34=16%
sufficient

2. 16% of the distribution is less than m-d
Since the distribution is symmetric, this tells us that m+d = 16%
sufficient.

The answer is D


I drew a pic, even if the distribution is not bell shaped, it still works

purple/pink is what we are looking for.