Assertion (A): The Rutherford and Thomson models both make up an unstable system.

Reason (R): Electrostatic instability makes Thomson's model unstable, whereas electromagnetic instability makes Rutherford's model unstable.

Both A and R are true and R is the correct explanation of A.

Thomson was unable to explain the how protons and electrons are holding together inside the atom. 

Rutherford was unable to explain the stability of atom as the electrons will undergo acceleration during rotation and release energy. Electrons will eventually lose their energy as a result of this and fall into the nucleus.