The core charge on oxygen is equal to $-$

Among the following,the energy of $2s$ orbital is lowest in

Arrange $s, p, d,$ and $f$ subshells of a shell in the increasing order of effective nuclear charge $(Z_{eff})$ experienced by the electron present in them.

The bromine atom possesses $35$ electrons. It contains $6$ electrons in $2p$ orbital,$6$ electrons in $3p$ orbital and $5$ electrons in $4p$ orbital. Which of these electrons experiences the lowest effective nuclear charge?

Between $2s$ and $2p$ electrons,which one experiences a greater shielding effect from inner shells?

Effective nuclear charge is maximum in case of