Solution:
Let Johnny's present age be x and Bobby's present age be y.
Half of Bobby's age is y/2.
Johnny must have been y/2 years of age (x-y/2) years ago.
At that time Bobby must have been y - (x - y/2) = 3y/2 - x years of age.
So x = 3y/2 - x
Or 2x = 3y/2.
Or 4x = 3y.

Consider first statement (1) alone.
When Johnny was born, Bobby must have been y - x years of age.
Or 4*(y-x) = y
Or 3y = 4x.
This is just the same equation as what we get from the main question.
So (1) alone is not sufficient.

Next consider (2) alone.
It means y - x = 6.
So we have 2 equations, 4x = 3y and y - x = 6.
Or x = 18 and y is 24.

So Bobby's age now is 24 years.

Since (2) alone is sufficient, (B) is the correct answer.