Explanation:

Negatively charged sol:
A sol is a colloidal suspension in which solid particles are dispersed in a liquid medium. The charge on the sol particles determines their behavior and interaction with other substances. A negatively charged sol refers to a sol in which the dispersed particles carry a negative charge.

Ag sol:
Ag sol refers to a sol in which the dispersed particles are made up of silver (Ag) atoms or ions. In an Ag sol, the silver particles are negatively charged. This charge arises due to the presence of excess electrons on the surface of the particles. The negative charge on the silver particles makes the sol negatively charged as a whole.

Reason for Ag sol being negatively charged:
The negative charge on the silver particles in an Ag sol can be attributed to several factors:

1. Ionization: When silver atoms or molecules are dispersed in a liquid medium, they can undergo ionization, resulting in the formation of Ag+ ions. These Ag+ ions carry a positive charge, leaving behind excess electrons on the surface of the particles, leading to a net negative charge.

2. Adsorption of ions: Ag sols can also acquire a negative charge through the adsorption of negatively charged ions from the surrounding medium. For example, if chloride ions (Cl-) are present in the medium, they can adsorb onto the surface of the silver particles, imparting a negative charge.

3. Electrostatic stabilization: In some cases, the negative charge on the sol particles may arise due to the presence of stabilizing agents or surfactants. These compounds can adsorb onto the particle surface and create a negatively charged layer, preventing the particles from aggregating or settling.

Conclusion:
In conclusion, the correct option for a negatively charged sol is option 'D' - Ag sol. This sol consists of silver particles that carry a negative charge, either due to ionization, adsorption of ions, or the presence of stabilizing agents. The negative charge on the sol particles plays a crucial role in determining their behavior and interactions with other substances.