**Answer:**

**Introduction:**
When two electronegative elements combine, they form a polar covalent bond. In such a bond, the electron density is unevenly distributed between the two atoms. The electronegative atom attracts the shared electrons more strongly, resulting in a partial negative charge on that atom and a partial positive charge on the other atom.

**Explanation:**
In the compound X-H.Y, where X and Y are both electronegative elements, the electron density distribution depends on the relative electronegativity of X and Y.

1. **Electron Density on X Increases and on H Decreases:**
- If X is more electronegative than Y, it will attract the shared electrons more strongly towards itself.
- This will result in an increase in electron density on X, as it gains partial negative charge.
- On the other hand, H will have a decrease in electron density, as it loses partial negative charge and gains partial positive charge.

2. **Electron Density on X and Y Increases:**
- If X and Y have similar electronegativities, they will have a similar ability to attract the shared electrons.
- In this case, both X and Y will have an increase in electron density, as they both gain partial negative charges.

3. **Electron Density on X and Y Decreases:**
- If X and Y are less electronegative than the central atom H, the shared electrons will be attracted towards H.
- As a result, both X and Y will experience a decrease in electron density, as they lose partial negative charges.

4. **Electron Density on X Decreases and on H Increases:**
- If Y is more electronegative than X, it will attract the shared electrons more strongly towards itself.
- This will result in a decrease in electron density on X, as it loses partial negative charge.
- On the other hand, H will have an increase in electron density, as it gains partial negative charge.

**Conclusion:**
The correct answer is option 1: The electron density on X will increase, and on H will decrease when X and Y are both electronegative elements. This is because the more electronegative element attracts the shared electrons more strongly, resulting in an uneven distribution of electron density.

As we know that electronegative element has tendency to attract shared pair of electrons so electron density on electronegative element will increase ,,, according to question in the question electron density will increase on X and Y while electron density will decrease on H