How is the atomic radius measured? Describe it.

The increasing order of the atomic radius of $Si, S, Na, Mg, Al$ is

Given below are two statements:
Statement $I$: The metallic radius of $Na$ is $1.86 \ \mathring{A}$ and the ionic radius of $Na^{+}$ is lesser than $1.86 \ \mathring{A}$.
Statement $II$: Ions are always smaller in size than the corresponding elements.
In the light of the above statements,choose the correct answer from the options given below:

Identify the correct order in which the covalent radius of the following elements increases: $(I) \ Ti, (II) \ Ca, (III) \ Sc$.

The largest element among $O$,$Se$,$S$,and $Te$ is:

The correct order of atomic radii of given elements is