The element of alkaline earth metal which shows a diagonal relationship with aluminium is:
(A) Na
(B) Ba
(C) Be
(D) Ca

Hint: Aluminium is the element of the 3rd group of the periodic table. So, you need to find an element that is in the 2nd group and situated diagonally to aluminium in the periodic table.

Complete step by step answer:
Now, we will discuss the answer stepwise -
Diagonal relationships exist between certain pairs of diagonally adjacent elements in the second and third periods (first 20 elements) of the periodic table.
Beryllium (Be) shows a diagonal relationship with aluminium.
This occurs due to the similar value of charge/radius ratio and electronegativity. Some of the resemblances are as follows:
Beryllium forms covalent compounds like aluminium (group-2 elements don’t have the tendency to form covalent bonds).
The hydroxides of beryllium and aluminium are amphoteric in nature, whereas those of other elements of a group-2 are basic in nature.
The oxides of both Be and Al (BeO and ${ Al }_{ 2 }{ O }_{ 3 }$) are high melting insoluble solids. ${ BeCl }_{ 2 }$ and ${ AlCl }_{ 3 }$ have bridged chloride polymeric structure.

Therefore, we can conclude that the correct answer to this question is option C.

Note: The other pairs of diagonal relationships are lithium (Li) and magnesium (Mg), boron (B), and silicon (Si), etc. They also exhibit similar properties. For example, the elements boron and silicon are both semiconductors.