[tex]^2^1^2_8_3Bi\;\rightarrow\;^2^1^2_8_4Po\;+\;^0_-1e[/tex] represents a beta decay. Parent isotope has 83 protons, daughter isotope has only one more proton (84). Beta particle is very small, atomic mass of 212 stays the same in beta decay.
What beta decay?
In nuclear physics, beta decay (β-decay) is a type of radioactive decay in which a beta particle (fast energetic electron or positron) is emitted from an atomic nucleus, transforming the original nuclide to an isobar of that nuclide.
[tex]^2^1^2_8_3Bi\;\rightarrow\;^2^1^2_8_4Po\;+\;^0_-1e[/tex]
In beta decay, one of the neutrons in the nucleus suddenly changes into a proton, causing an increase in the atomic number of an element.
Hence, option D is correct.
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