Watson and Crick (1953) proposed DNA was based on
(i) X-ray diffraction studies studies of DNA dne by Wilkins and Franklin
(ii) Chargaff's base equivalence rule
(iii)Griffith's transformation experiment
(iv) Meselson and Stahl's experiment.

Options

A. (i),(ii) and (iv)
B. (i) and (ii)
C. (iii) and (iv)
D. (i), (ii),(iii) and (iv)

Explanation

To solve the question regarding the basis of Watson and Crick's proposal of the DNA structure in 1953, we will analyze each option provided:

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks which studies or experiments Watson and Crick based their DNA model on. We need to evaluate each of the four options given.

2. **Option (i): X-ray diffraction studies of DNA by Wilkins and Franklin**:
- X-ray diffraction studies conducted by Rosalind Franklin and Maurice Wilkins provided crucial evidence for the helical structure of DNA. Franklin's famous Photo 51 was instrumental in revealing the double helix shape of DNA. Thus, this option is correct.

3. **Option (ii): Chargaff's base equivalence rule**:
- Chargaff's rules state that in any given DNA molecule, the amount of adenine (A) is equal to thymine (T), and the amount of guanine (G) is equal to cytosine (C). This base pairing was fundamental in the development of the double helix model proposed by Watson and Crick. Hence, this option is also correct.

4. **Option (iii): Griffith's transformation experiment**:
- Griffith's experiment demonstrated the phenomenon of transformation, where non-virulent bacteria could become virulent by taking up genetic material from heat-killed virulent bacteria. While significant in the history of genetics, it did not directly contribute to the structure of DNA as proposed by Watson and Crick. Therefore, this option is incorrect.

5. **Option (iv): Meselson and Stahl's experiment**:
- The Meselson and Stahl experiment, conducted in 1958, provided evidence for the semi-conservative replication of DNA. This experiment came after Watson and Crick proposed their model and thus is not relevant to the basis of their proposal. Hence, this option is also incorrect.

6. **Conclusion**: Based on the analysis, the correct answers are options (i) and (ii). Therefore, the best choice reflecting the basis of Watson and Crick's DNA model is option B, which includes both X-ray diffraction studies and Chargaff's base equivalence rule.

### Final Answer:
The model proposed by Watson and Crick was based on:
- (i) X-ray diffraction studies of DNA done by Wilkins and Franklin
- (ii) Chargaff's base equivalence rule