Final answer:
In double-stranded DNA, the C content is always equal to the G content and the A content is always equal to the T content due to base pairing. Therefore, if C is 20%, T is also 20%.
Step-by-step explanation:
If the C content of a preparation of double-stranded DNA is 20%, according to Chargaff's rules, the T content would also be 20%. Chargaff's rules state that DNA is composed of base pairs with adenine (A) pairing with thymine (T) and cytosine (C) pairing with guanine (G). Therefore, the amount of cytosine will always be equal to the amount of guanine, and the amount of adenine will always be equal to the amount of thymine. Consequently, if the C content is 20%, the G content will also be 20%, and the remaining 60% of the DNA will be divided equally between adenine and thymine, giving us 30% A and 30% T. However, as we are specifically informed that the C content is 20%, the T content must also be 20%, making the correct answer B) 20%.