Periods on the periodic table are arranged
a. in horizontal rows.
b. by order of density.
c. in vertical columns.
d. in no particular order.

The correct answer is A. In horizontal rows

Explanation:

In the periodic table, the term "periods" refers to the horizontal rows; this is the opposite of "groups" that refer to the columns or vertical sections in the periodic table. In total there are seven different rows, and each row contains elements of different groups. Moreover, in each row, the elements are organized according to the atomic number (number of protons) and each row refers to a specific electron shell configuration (energy level). For example, in the fourth row, the first element is Potassium (K) and the atomic number of this is 19, while the last element of this period or row is Krypton (Kr) and the atomic number of this is 36. Also, all the elements in this row have the same number of electrons or electron configuration. According to this, the periods are arranged in horizontal rows.