The valency of carbon is $4$. On what principle can it be explained in a better way?

The types of hybridisation on the five carbon atoms from left to right in $pent-1-en-4-yne$ are

The ion that has $sp^3d^2$ hybridization for the central atom is

Predict the hybridization of each carbon in the molecule of the organic compound given below. Also,indicate the total number of sigma $(\sigma)$ and pi $(\pi)$ bonds in this molecule.
$HC \equiv C-CO-CH_2-COOH$

The species in which the nitrogen atom is in a state of $sp$ hybridisation is

In the given reaction,the number of $sp^{2}$ hybridised carbon atoms in compound '$X$' is $.....$