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.