The correct answer is option 4, Boric acid. A colloid is a type of mixture in which one substance is dispersed evenly throughout another substance. It consists of two phases - a dispersed phase and a continuous phase. The dispersed phase is made up of tiny particles, called colloidal particles, that are evenly distributed throughout the continuous phase.

Now, let`s analyze each option to understand why option 4 is not a colloid:

Option 1: Milk - Milk is a colloid. It contains fat globules dispersed throughout water.

Option 2: Mud - Mud is also a colloid. It consists of tiny solid particles suspended in water.

Option 3: Butter - Butter is a colloid as well. It is made up of water droplets dispersed in a continuous fat phase.

Option 4: Boric acid - Boric acid is not a colloid. It is a solid substance that does not disperse into smaller particles when mixed with another substance.

In conclusion, option 4, Boric acid, is not a colloid because it does not consist of dispersed particles in a continuous phase.