16) He had already left .... the early bus to Pune.
Explanation: 'on' is used for public modes of transportation, e.g. on the bus, on the plane, on a ship, etc.
17) The dog is lying ..... the floor.
Explanation: 'on' is used for surfaces.
18) Jack is playing volleyball with his friends .... the beach.
Explanation: 'on' is a preposition of the location which is used for smaller and more specific locations or places.
19) She will meet you ...... the store.
Explanation: 'at' is used for the smallest and most specific places, e.g. address, a specific place or point such as at 350 Sunview Street, at the store, at my house, etc.
20) She received lots of gifts .... her birthday.
Explanation: 'on' is used for days, or shorter periods of time, e.g., on his birthday, on Monday, on the weekend, etc.