Answer
A. 20 percent
Work Step by Step
In DNA (double stranded), the amount of guanine units (G) is equal to the amount of cytosine units (C). The amount of adenine units (A) is equal to the amount of thymine units (T).
The sum of A and G (purines) is equal to the sum of C and T (pyrimidines).
If the percentage of Adenine is 30%, then the percentage of Thymine is also 30%. The sum of A and T is 60% and remaining is 40% which has to be divided equally (between G and C). Thus guanine (G) has 20 percent.
