Study of radioactive decay of unstable nuclei resulting in emission of a particle lead us to four different observations listed below:

(A) no change in atomic number and mass number.
(B) no change in mass number but the atomic number decreases by 1.
(C) no change in mass number but the atomic number increases by 1.
(D) atomic number decreases by 2 and the mass number decreases by 4.

Arrange the particles emitted in the above four cases in correct sequence.
Choose the correct answer from the options given below.

photon, positron, electron, alpha particle.

The correct answer is Option (3) → photon, positron, electron, alpha particle.

(A) This corresponds to ($γ$) decay, where only energy is emitted as photons, and there is no change in the nucleus.

(B) This corresponds to beta-plus $(β^+)$ decay or positron emission, where a proton is converted into neutron, and the atomic number decreases by 1.

(C) This corresponds to beta-minus $(β^-)$ decay, where a neutron is converted into proton and the atomic number increases 1.

(D) This corresponds to Alpha ($α$)-decay, where a helium nucleus is emitted.