An atom is an smallest particle of the covalent compound that has the properties of compound.
Atom is the smallest particle that take part in chemical reaction and atoms are made up of particles called electrons, protons and neutrons, which is responsible for the charge of the atoms.
The smallest unit of compound is called molecule. Molecules are made up of atoms which is connected by bond by sharing of electrons and molecules have also the properties of compound.
Sand would definitely increase friction because of the roughness of it surface. When its surface comes with another rough surface it would tend to increase friction. All other options given tend to reduce friction one way or the other.
