a. The bases which are soluble in water are called alkali.

Sodium hydroxide is a base which is soluble in water and hence it is an alkali.

b. A compound is said to be acidic in character if it is able to generate H⁺ ions in aqueous solutions. HCl, HNO₃, etc. show acidic characters in aqueous solutions because they are able to generate H⁺ ions in aqueous solutions while solutions of compounds like alcohol and glucose do not show acidic character because they are not able to generate H⁺ ions in aqueous solutions