A monoatomic anion $$(A^-)$$ has 45 neutrons and 36 electrons. Atomic mass, group in the periodic table and physical state at room temperature of the element $$(A)$$ respectively are :

The monoatomic anion is $$A^-$$ and it is given that it possesses $$36$$ electrons.

For any species,
$$\text{number of electrons} = Z - (\text{charge})$$.
Since the charge on $$A^-$$ is $$-1$$, we have
$$36 = Z + 1 \; \; \Rightarrow \; \; Z = 35$$.

Atomic number $$35$$ corresponds to the element bromine (symbol $$Br$$).

The anion contains $$45$$ neutrons. For any nuclide,
$$\text{mass number } (A) = Z + N$$,
where $$N$$ is the number of neutrons. Hence
$$A = 35 + 45 = 80$$.

Bromine belongs to the halogen family, i.e. Group $$17$$ of the periodic table.

At room temperature (≈298 K), elemental bromine exists as a reddish-brown liquid. It is one of only two elements that are liquids under these conditions (the other being mercury).

Therefore the required set {atomic mass, group, physical state} is $$80, 17, \text{liquid}$$.

Option A which is: $$80, 17, \text{liquid}$$