Fill in the blank with the most appropriate phrase.

When the lockdown was over, people were ______ work.

The correct answer is - "back to" 

Key Points

• From the given options, the most appropriate answer is "Back to" as it fits with the context and will become grammatically correct.
• Let us see the meanings of the given phrase in the options -
  • Back with - to return with something or someone.
    • Example - He will be back with his teammates.
  • Back to - to return from where one left earlier..
    • Example - He never back to him home after family skirmishes.
  • Back up - have a support on failing.
    • Example - I needed power back up in case the light went off.

 So the correct option is Option 2