1. Check Assertion A

Water has the molecular formula $\mathrm{H_2O}$. Around oxygen, there are:

• $2$ bond pairs
• $2$ lone pairs

According to VSEPR theory, the electron pair geometry is tetrahedral, but because of the two lone pairs, the molecular shape is bent (angular).

For water, the observed bond angle is: $\angle \mathrm{H-O-H} = 104.5^\circ$ So, Assertion A is true.

2. Check Reason R

By VSEPR theory, electron pair repulsions follow the order: $\text{lone pair-lone pair} > \text{lone pair-bond pair} > \text{bond pair-bond pair}$ Thus, lone pair-lone pair repulsion is greater than bond pair-bond pair repulsion. So, Reason R is true.

3. Does R correctly explain A?

In an ideal tetrahedral arrangement, the angle is: $109.5^\circ$ But in water, oxygen has two lone pairs. Since lone pairs repel more strongly than bond pairs, they compress the bond angle between the two $\mathrm{O-H}$ bonds from $109.5^\circ$ to $104.5^\circ$.

Hence, the reason correctly explains why water has a bond angle of $104.5^\circ$.

4. Conclusion

• Assertion A is true.
• Reason R is true.
• Reason R correctly explains Assertion A.

Therefore, the correct option is: $\boxed{A}$