The electronegativities of four elements A, B, C, D are 4,3.17, 0.7 and 2.85 respectively and the electronegativities of hydrogen (H) and oxygen (O) atoms are 2.1 and 3.5 respectively.
Which of the following compounds does not act as acid? 

Options

A. H-O-A
B. H-O-B
C. H-O-C
D. H-O-D

Answer

To determine which of the compounds formed by elements A, B, C, and D does not act as an acid, we need to analyze the electronegativities of these elements in relation to hydrogen (H) and oxygen (O).

### Step-by-Step Solution:

1. **Identify the Electronegativity Values**:
- Element A: 4.0
- Element B: 3.17
- Element C: 0.7
- Element D: 2.85
- Hydrogen (H): 2.1
- Oxygen (O): 3.5

2. **Understand Acid Behavior**:
- A compound acts as an acid if it can easily donate a proton (H⁺). This typically occurs when there is a significant electronegativity difference between hydrogen and the other element in the compound. A higher electronegativity of the other element compared to hydrogen means that the bond is more polar, making it easier for H⁺ to be released.

3. **Calculate the Electronegativity Differences**:
- For A (4.0) and H (2.1): Difference = 4.0 - 2.1 = 1.9
- For B (3.17) and H (2.1): Difference = 3.17 - 2.1 = 1.07
- For C (0.7) and H (2.1): Difference = 2.1 - 0.7 = 1.4
- For D (2.85) and H (2.1): Difference = 2.85 - 2.1 = 0.75

4. **Analyze the Results**:
- The larger the electronegativity difference, the more likely the compound will behave as an acid.
- A (1.9) and B (1.07) have significant differences, indicating they can act as acids.
- C (1.4) also has a considerable difference but is lower than A and B.
- D (0.75) has the smallest difference, suggesting it is the least likely to donate H⁺.

5. **Conclusion**:
- Since D has the smallest electronegativity difference with hydrogen, it is the least likely to act as an acid. Therefore, the compound formed with element D does not act as an acid.

### Final Answer:
The compound that does not act as an acid is formed by element **D**.