**Core Concept**
Chargaff's base pair rule states that in DNA, the amount of adenine (A) is equal to the amount of thymine (T), and the amount of guanine (G) is equal to the amount of cytosine (C). This means that the sum of A and G, as well as the sum of T and C, are always equal.

**Why the Correct Answer is Right**
Given that the content of base pair A is 15%, we know that the content of base pair T must also be 15%. Since A pairs with T, and G pairs with C, we can infer that the content of base pair G must be equal to the content of base pair C. Let's assume the content of base pair G is x%. According to Chargaff's rule, the content of base pair C must also be x%. Since the total content of the four bases must add up to 100%, we can write the equation: A + G + T + C = 100%. Substituting the values we know, we get 15% + x% + 15% + x% = 100%. Combining like terms, we get 30% + 2x% = 100%. Solving for x, we get 2x% = 70%, or x% = 35%. Therefore, the content of base pair G is 35%.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the content of base pair A is given as 15%, but the content of base pair G cannot be 15% since G pairs with C, not T.

**Option B:** This option is incorrect because the content of base pair G is not equal to the content of base pair T, which is 15%.

**Option D:** This option is incorrect because the content of base pair G is not equal to the sum of the content of base pair A and the content of base pair T, which is 30%.

**Clinical Pearl / High-Yield Fact**
Chargaff's base pair rule is a fundamental principle in molecular biology that has been used to determine the sequence of DNA molecules. It is essential to remember that the sum of the content of base pairs A and G, as well as the sum of the content of base pairs T and C, are always equal.

**✓ Correct Answer: C. 35%**