The following ratio is generally constant for a given species

Options

A. A+G/C+T
B. T+C/G+A
C. G+C/A+T
D. A+C/T+G

Explanation

To solve the question regarding the constant ratio for a given species, we will follow these steps:

### Step 1: Understand the Components of DNA
DNA is composed of four nitrogenous bases: Adenine (A), Thymine (T), Cytosine (C), and Guanine (G). These bases are categorized into two groups:
- **Purines**: Adenine (A) and Guanine (G)
- **Pyrimidines**: Cytosine (C) and Thymine (T)

### Step 2: Identify Chargaff's Rules
Erwin Chargaff proposed certain rules in 1950 based on his observations of the base composition of DNA. The key rules are:
1. The molar ratio of purines (A + G) to pyrimidines (T + C) is constant for a given species.
2. The amounts of adenine (A) are equal to thymine (T), and the amounts of guanine (G) are equal to cytosine (C). This can be expressed as:
- A = T
- G = C

### Step 3: Analyze the Ratios
From Chargaff's rules, we can derive the following ratios:
- The ratio of A + T to G + C can be expressed as:
\[ \frac{A + T}{G + C} = \text{constant for a species} \]
- However, the individual ratios of A to T and G to C may vary among different organisms.

### Step 4: Determine the Correct Ratio
The question asks for the ratio that is generally constant for a given species. Based on Chargaff's rules, the correct ratio is:
\[ \frac{G + C}{A + T} = \text{constant for a species} \]
This means that while the individual ratios may vary, the overall ratio of G + C to A + T remains constant within a species.

### Conclusion
Thus, the answer to the question is that the ratio \( G + C \) to \( A + T \) is generally constant for a given species.

---