Answer:
A. 4s subshell.
B. An element with the valence electron configuration 2s² 2p⁵ would form a monatomic ion with a charge of -1.
In order to form this ion, the element will gain 1 electron into the 2p subshell.
Explanation:
A. An element with the valence electron configuration 4s² 3d⁶ is a transition metal. The electrons lost will be the ones from the last level (4), that is, the 4s subshell. The electron configuration of the ion will be 4s⁰ 3d⁶.
B. An element with the valence electron configuration 2s² 2p⁵ is a nonmetal with 2 + 5 = 7 electrons in its valence shell. To complete its octet, it will gain 1 electron into the subshell 2p. Now having an extra electron, it will have a charge of -1.
