The police arrested the thief but his accomplice escaped.

The correct answer is: accomplice.

An accomplice is someone who helps another person commit a crime. In the given question, the thief’s accomplice escaped from the police.

A friend is someone who you like and enjoy spending time with. A colleague is someone who you work with. A companion is someone who you travel with. A partner is someone who you are in a romantic relationship with.

In the given question, the thief’s accomplice is not a friend, colleague, companion, or partner. The accomplice is someone who helps the thief commit a crime.