periods on the periodic table are arranged
oa) in horizontal rows.
ob) by order of density.
oc) in vertical columns
od) in no particular order.

A) in horizontal rows  

Explanation:

The Periodic Table is arranged into 18 vertical Groups and seven horizontal Rows or Periods.

For example, the red arrow in the picture below shows Row 4 or the 4th Period.

B) and D) are wrong. The Periodic Table is arranged according to increasing  atomic number.

C) is wrong. The Groups are arranged in vertical columns.

the number of valence electrons for each molecule or ion is shown beneath the structure. remember hydrogen will not have more than two electrons. this hydrogen is part of a covalent bond (sharing two electrons). a double bond here would cause hydrogen to share four electrons with phosphorus.