Which of the following is a true statement? a. Periods form horizontal rows; groups form vertical columns
b. Groups form horizontal rows; periods form vertical columns
c. Categories form horizontal rows; gases form vertical columns
d. Gases form horizontal rows; categories form vertical columns

a. Periods form horizontal rows; groups form vertical columns

Explanation:

On the periodic table of elements, it is true that periods form horizontal rows and groups form vertical columns.

In the periodic table, elements having the same number of electrons in the outermost shell of their atoms are placed over one another in vertical columns.