Assertion: In alpha decay of different radioactive nuclides, energy of alpha particles has been compared. It is found that as the energy of alpha particles increases the half life of the decay decreases.
Reason: More is the energy in any decay process; more is the probability of decaying the nuclide which leads to faster rate of decay.
(A). Both assertion and reason are correct and reason is the correct explanation for assertion
(B). Both assertion and reason are correct but reason is not the correct explanation for assertion
(C). Assertion is correct but reason is incorrect
(D). Both assertion and reason are incorrect

Hint: When a radioactive substance undergoes decay, it emits different types of particles and its chemical state changes. In alpha decay, alpha particles are emitted. Alpha particles are similar to helium. The more unstable a nucleus is, the faster it decays and the more it emits particles.

Complete answer:
A radioactive substance is a highly unstable atom which undergoes three types of decay; alpha, beta and gamma decay.
In alpha decay, alpha particles are emitted which are helium like atoms with a positive charge.
Half life is the time taken by a radioactive substance to reduce to half of its amount by radioactive processes.
The higher the energy of a nucleus, the more unstable it is and hence undergoes radioactive decay at a faster rate. Therefore, when a nucleus possesses higher energy, it emits alpha particles with a faster rate in the alpha decay process.
As the rate of decay increases, the half life of the substance also decreases. So we can say that, the assertion and reason is correct and reason is the correct explanation.
Therefore, the assertion and reason both are correct and reason explains the assertion correctly.

Hence, the correct option is (A).

Note:
Beta particles are similar to electrons while gamma particles are electromagnetic waves. The alpha particles have the highest penetrating power. When alpha decay takes place, the atomic number increases by two and the mass number decreases by four. The stability of a radioactive substance is determined by the binding energy per nucleon.