Prob of picking any marble = 1

After picking the first marble, you are left with 5 marbles. Only 1 marble from these 5 will match the first one. Therefore:

Prob(2 same colored marbles) = 1 x (1/5) = 1/5

Choose D.

-BM-

An alternate solution:

Total number of outcomes = Total ways of selecting 2 marbles from the given 6 marbles = 6C2 = 15

Desired outcomes = 2 balls of same color can be selected in following three ways:
Select both red balls = 2C2
Select both blue balls = 2C2
Select both green balls = 2C2

Now at a time we can either select both red balls OR both blue balls OR both green balls, so the desired number of outcomes = 2C2+2C2+2C2 = 1+1+1= 3

So probability = 3/15 = 1/5