Final answer:
Using Chargaff's rule and the information that there are 50 base pairs with 15 cytosines, we deduce there must also be 15 guanines. With 100 total bases, and 30 taken by C and G, we're left with 70, which must be equally adenines and thymines, concluding that there are 35 thymine bases.
Step-by-step explanation:
If a DNA molecule of 50 base pairs contains 15 cytosine bases (C), it's important to use Chargaff's rule to help answer the question regarding the number of thymine bases (T). Chargaff's rule states that in any double-stranded DNA, the number of guanine units (G) equals the number of cytosine units (C), and the number of adenine units (A) equals the number of thymine units (T). Since the DNA strand has 15 cytosine bases, it must also have 15 guanine bases to comply with Chargaff's rules.
Knowing that a DNA molecule consists of pairs, and that there are 50 base pairs, we can calculate the total number of bases which is 100 (50 pairs × 2 bases per pair). If there are 15 cytosines and consequently 15 guanines, this accounts for 30 bases (15 C + 15 G). That leaves 70 bases, which must be divided equally between adenine and thymine because they pair with each other.
Therefore, there are 35 adenine bases and 35 thymine bases. So the correct answer is 35 thymine bases, which corresponds to choice (d).