Step 1: Understand what is asked.
A sol is a colloid where tiny solid particles float in a liquid. We must find which one carries a negative charge.
Step 2: Know why sols carry charge.
The tiny particles soak up ions from the liquid around them. This gives each particle either a positive or a negative charge.
Step 3: List the positive ones.
Metal oxide sols like $\text{TiO}_2$, hydrated oxides like $\text{Fe}_2\text{O}_3 \cdot x\text{H}_2\text{O}$, and blood proteins like haemoglobin are all positively charged.
Step 4: List the negative ones.
Sols of clay, charcoal, metal sulfides, and metals like gold or silver are negatively charged.
Step 5: Match our options.
Three of the choices are in the positive group. Only the sol of clay is in the negative group.
Step 6: Conclusion.
Natural soil and mud sols, like clay, are negatively charged.
\[ \boxed{\text{Sol of clay (Option 1)}} \]