The periodic table is composed of and
a. periods and rows
b. groups and columns
c. molecules and groups
d. groups and periods

D. groups and periods

Explanation:

In groups  the elements have the same electronic configuration in the last electronic shell and we expect that they will have similar chemical properties.

In periods the electronic configuration is increasing with one electron as we proceed from left to right and so the chemical properties will vary significantly.

a:

by the combination of an unlimited number of elements