Cytosine is present?
The complementary base-pairing rule states that Adenine always pairs with Thymine (A with T), and Cytosine with Guanine (C with G).
Because of this, the amount of Adenine present must be the same as the amount of Thymine present, and the same for Cytosine and Guanine.
This means that if there is 30% Adenine, there is also 30% Thymine as each A base is paired to a T base. This adds to 60%, so Cytosine amounts and Guanine amounts must add to 40% to make 100%.
Since they are equal in amount, there must be 20% of each.
Therefore the answer is B. 20%
There is 30% Adenine, 30% Thymine, 20% Guanine and 20% Cytosine.
Hope this helped!