Final answer:
According to Chargaff's rules, a DNA strand with 30% thymine (T) will also have 30% adenine (A) due to A-T base pairing. The correct option for the percentage of A is therefore 30%, option b).
Step-by-step explanation:
If a DNA strand consists of 30% thymine (T), then it would have 30% adenine (A) as well. This is because in DNA, the nitrogenous bases pair up in a specific way: adenine (A) pairs with thymine (T), and guanine (G) pairs with cytosine (C).
This is known as base pairing, which was elucidated by Erwin Chargaff, and is commonly referred to as Chargaff's rules. According to these rules, the amount of adenine in a DNA molecule is always equal to the amount of thymine, and the amount of guanine is always equal to the amount of cytosine.
Thus, if a DNA strand has 30% T, the same strand must have an equal percentage of A to form the A-T base pair. This would mean the correct option for the percentage of A is 30%, which is option b) of the multiple choices provided.