Which one is the electron deficient compound?

Which atom can have more than eight valence electrons when it is forming covalent bonds?

Multiple covalent bonds exist in a molecule of

Who gave the concept of a covalent bond? What is a covalent bond? Explain with one example.

If a molecule $X_2$ has a triple bond,then $X$ will have the electronic configuration

The electronic configuration of three elements $A$,$B$ and $C$ are given below. Stable form of $C$ may be represented by the formula
$A : 1s^2, 2s^2, 2p^6$
$B : 1s^2, 2s^2, 2p^6, 3s^2, 3p^3$
$C : 1s^2, 2s^2, 2p^6, 3s^2, 3p^5$