**Core Concept**
The question is testing the student's understanding of the base composition of double-stranded DNA (dsDNA). In dsDNA, the base pairing rules dictate that adenine (A) pairs with thymine (T) and cytosine (C) pairs with guanine (G). The base composition of dsDNA is typically expressed as a percentage of the total bases.

**Why the Correct Answer is Right**
Given that the molar ratio of Adenosine (A) is 20%, and knowing that A pairs with T, we can infer that the molar ratio of Thymine (T) is also 20%. Since the total base composition must add up to 100%, we can calculate the molar ratio of Cytosine (C) by subtracting the sum of A and T from 100%. Assuming a neutral GC content (i.e., no bias towards either G or C), we can infer that the molar ratio of Cytosine (C) is approximately 30%.

**Why Each Wrong Option is Incorrect**
* **Option A:** Assuming a neutral GC content, Option A's molar ratio of Cytosine (C) is too high and doesn't account for the base pairing rules.
* **Option B:** Option B's molar ratio of Cytosine (C) is too low and doesn't take into account the complementary nature of A and T.

**Clinical Pearl / High-Yield Fact**
In general, the base composition of dsDNA is typically around 20% A, 20% T, 30% C, and 30% G. However, this can vary depending on the organism and the specific DNA sequence.

**Correct Answer: C. 30%**