In general, atomic radii:

A. decrease down a group and remain constant across a period.

B. decrease down a group and increase across a period.

C. increase down a group and increase across a period.

D. increase down a group and remain constant across a period.

E. increase down a group and decrease across a period.

Answer: The correct answer is Option E.

Explanation:

Atomic radius is defined as the total distance measured from the nucleus of an atom to the outermost shell.

Trend down the group:

Moving from top to bottom, a new shell gets add up around the nucleus and the outermost shell gets far away from the nucleus. Due to this, the distance between the nucleus and outermost shell increases, which results in the increase of atomic radii of the atom.

Trend across the period:

Moving from left to right in a period, more and more electrons gets add up in the same shell. The attraction between the last electron and the nucleus increases. This results in the shrinkage of size of an atom. Thus, decreasing the atomic radii of the atom.

Hence, the correct answer is Option E.