Matter exists in three physical forms -solid, liquid and gas. In which state the force of attraction between the molecules is (a)strongest (b)weakest?

There are intra-molecular forces that keep the molecule together. 

There are different kinds of forces like Van der Waals forces, electrostatic forces, magnetic forces, and forces that lead to the formation of ionic and covalent bonds.

These intermolecular forces are strongest in solids where the molecules are densely packed due to these forces and are weakest in gases where the molecules are sparsely packed.