If a double-stranded DNA has 15% of adenine, find out the percent of cytosine in the DNA?

The correct option is 35%.

According to Chargaff's rules of base pairing in a double-stranded DNA molecule, the total amount of purines is always equal to the total amount of pyrimidines. Specifically:
1. Adenine (A) pairs with Thymine (T), which means:
$$A=T$$ 2. Guanine (G) pairs with Cytosine (C), which means:
$$G=C$$

The sum of all four nitrogenous bases in the DNA is 100%:
$$A+T+G+C=100\%$$

Given in the problem, Adenine (A) is equal to 15%.
Using Chargaff's rule:
$$T=A=15\%$$

Therefore, the combined percentage of Adenine and Thymine is:
$$A+T=15\%+15\%=30\%$$

Next, we find the remaining percentage for Guanine and Cytosine:
$$G+C=100\%-30\%=70\%$$

Since the percentage of Guanine is equal to the percentage of Cytosine, we divide the remaining percentage by 2:
$$C=\frac{70\%}{2}=35\%$$

Thus, the percentage of cytosine in the DNA is 35%.