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.