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.

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"among the transition metal elements, manganese and zinc have a slight lowering of electronegativities. the reason behind this is that the nuclear charges of these elements are slightly weaker than the other transition metal elements due to the way their valence electrons are filled up. both manganese and zinc have their valence electrons filled up to the 4th shell with 1 pair of electrons (4s2) completely occupying it. although the electrons in the 3rd shell (3d) are yet to be paired since the outer most shell is filled up, the ability of the nucleus to attract electrons is lessened. hence,   the electronegativity (or the ability to attract electrons) is lessened."