which element has the smallest atomic radius? calcium potassium scandium titanium

Brief Explanations:

Atomic radius decreases across a period. Calcium (Ca), potassium (K), scandium (Sc), and titanium (Ti) are in the same period (period 4). Potassium is in group 1, calcium in group 2, scandium in group 3 and titanium in group 4. As we move from left - to - right across a period, the effective nuclear charge increases, pulling the electrons closer to the nucleus, thus reducing the atomic radius.

Answer:

D. titanium