Question

what causes atoms to be pulled closer to the nucleus as you move across a period? reduced orbital size increased electron - electron repulsion decreased electron density increased positive nuclear charge

Explanation:

As you move across a period, the number of protons in the nucleus (positive nuclear charge) increases. This stronger positive charge pulls the electrons closer to the nucleus. Decreased electron - density would not pull atoms closer. Increased electron - electron repulsion pushes electrons apart. Reduced orbital size is an effect, not a cause.

Answer:

Increased positive nuclear charge