Which two elements are in the same period? a. nitrogen and phosphorus b. cobalt and iridium c. sodium and sulfur d. boron and silicon

The answer is C.

Elements are in the same period if they have the same number of occupied electron shells. Therefore, to find which 2 elements are in the same period, we first have to find each of their number of occupied electron shells. 

Nitrogen: Has 7 electrons, configuration is 2,5 (2 occupied electron shells)
Phosphorus: Has 15 electrons, configuration is 2, 8, 5 (3 occupied electron shells)
Answer A is incorrect.

Cobalt: Has 27 electrons, configuration is 2,8,15,2 (4 occupied shells)
Iridium: has 77 electrons, configuration is 2,8,18,32,15,2 (6 occupied shells)
B is incorrect

Sodium: Has 11 electrons, configuration is 2,8,1 (3 occupied shells)
Sulfur: has 16 electrons, configuration is 2,8,6 (3 occupied shells)

Therefore, as Sodium and Sulfur have the same number of occupied electron shells, they are in the same period. The answer is C.