16) He said, "Don't worry; it is a small problem."
Explanation: 'small' correctly describes the noun 'problem'
17) How many candies did she put into her pocket?
Explanation: 'many' correctly describes the noun 'candies'
18) She bought blue water bottles for the kids.
Explanation: 'blue' correctly describes the noun 'bottles'
19) He is not coming to the park for several days.
Explanation: 'several' is a numeral adjective that modifies the noun 'days'
20) This book on the table belongs to her.
Explanation: 'This' is a demonstrative adjective. It is used to indicate or demonstrate specific things.