An element with atomic number 21 is a:
(A) Halogen
(B) Representative element
(C) Transition element
(D) Alkali element

Hint: The atomic number gives the number of electrons present in the element, which are filled in the orbital around the nucleus following the Aufbau principle. In the configuration obtained, the filling of the last electron determines the nature of the element and the block to which it belongs.

Complete step by step solution:
Given the atomic number of the element to be 21. We get the electronic configuration to be $1{{s}^{2}}2{{s}^{2}}2{{p}^{6}}3{{s}^{2}}3{{p}^{6}}3{{d}^{1}}4{{s}^{2}}$, following the Aufbau principle and the (n + l) rule. The last electron enters the 3d- orbital. The element is Scandium, having partially- filled d-orbitals belonging to the d-block series.
The transition elements are the elements having their atoms with incomplete d-orbitals or their ions in their stable oxidation state have partially -filled d-orbitals.
Thus, making the Scandium a transition element with its atom having partially filled d-orbital. Even though it’s in $S{{c}^{2+}}$, it has an empty d-orbital.

Therefore, the element with atomic number 21 is option (D)- a transition element.

Additional information:
The halogens are the Group 17 elements having one less electron in their outermost shell belongs to the p-block. Whereas, the Group 1 element makes the alkali element with only one electron in the outermost s-orbital.
Both the s-block and p-block elements except d-block form the representative elements as their valence electrons denote the element in their respective groups.

Note: The transition elements consist of the d-block and f-block elements. The f-block elements also known as the inner-transition elements. Their properties and nature show the transition between the s-block and the p-block elements.
The Zn, Cd, and Hg elements are not classified as transition elements because they have completely filled d-subshell both in the atom or the ionic stable oxidation state.