Use of into is incorrect.
The chief guest entered the room is the correct sentence.

Correct Option: B

Use of into is incorrect.
The chief guest entered the room is the correct sentence.

On will replace At.
On his return is the appropriate usage.

Correct Option: A

On will replace At.
On his return is the appropriate usage.

Use of to (Prep.) is incorrect. So, it will not be used.
Hence, the function will replace to the function

Correct Option: C

Use of to (Prep.) is incorrect. So, it will not be used.
Hence, the function will replace to the function

Most of the people will replace Most people because –
When we talk about a large, general group, we use most and when we talk about a smaller, more specific group, we use most of.
Look at the examples given below :
(i) Most doctors earn a lot of money.
(ii) Most of the doctors here earn a lot of money.

Correct Option: A

Most of the people will replace Most people because –
When we talk about a large, general group, we use most and when we talk about a smaller, more specific group, we use most of.
Look at the examples given below :
(i) Most doctors earn a lot of money.
(ii) Most of the doctors here earn a lot of money.

furniture will replace furnitures because furniture is the plural of furniture
Hence, The furniture is the right usage

Correct Option: A

furniture will replace furnitures because furniture is the plural of furniture
Hence, The furniture is the right usage