(i) False, in elastic collision the linear momentum and kinetic energy of the system as a whole is conserved, but momentum and kinetic energy of each of individual body change.
(ii) False, internal as well as external forces can change the kinetic energy. Again forces if conservative may change the potential energy of a system.
(iii) False, for non-conservative force the work done over a closed
loop is not zero.
(iv) It is usually true but not always true. As an example in the explosion of a cracker final kinetic energy is greater than the initial kinetic energy.
Again final kinetic energy of gun-bullet system after firing is more than initial kinetic energy before collision.