Assertion (A) : Mercurous ion is paramagnetic.
Reason (R ): It contains one unpaired electron.

Options

A. If both Assertion (A) and (R ) are correct and Reason (R ) is the correct explanation of Assertion (A).
B. If both (A) and (R ) are correct but (R ) is not the correct explanation for (A).
C. If (A) is correct but (R ) is not correct.
D. If both (A) and (R ) are incorrect

Explanation

Both (A) and (R ) are false. Mercurous ion is `Hg_(2)^(2+)`.
`Hg(Z = 80) rArr 5d^(10) 4s^(2)`
`Hg^(o+) rArr 5d^(10) 4s^(1)`
It should be paramagnetic, containing one unpaired electron. But it was observed experimentally to be diamagnetic.
That is why two `Hg^(o+)` ions combines to give `Hg_(2)^(2+)`, so it become diamagnetic and does not contain any unpaired electron.