Question

what causes the shielding effect to remain constant across a period?
electrons are added to the same principal energy level.
the atomic radius increases.
the charge on the nucleus is constant.
electrons are added to different principal energy levels.

Explanation:

The shielding effect is related to the number of inner - shell electrons. When electrons are added to the same principal energy level (across a period), the number of inner - shell electrons (which cause shielding) remains the same. The atomic radius actually decreases across a period (so that option is wrong). The charge on the nucleus increases across a period (so that option is wrong). If electrons were added to different principal energy levels (like down a group), the shielding effect would change.

Answer:

Electrons are added to the same principal energy level.