Ratan Thyiam, the famous personality from Manipur is known for his work in the field of theatre. Theatre is a form of performing art that involves live performances of actors on stage, presenting a story or entertainment to an audience. It encompasses various elements such as acting, directing, set design, and costume design.

Option 1, sports, refers to competitive physical activities that require skill and physical exertion. There is no information or evidence to suggest that Ratan Thyiam is known for his work in sports.

Option 2, music, refers to the art of combining sounds and tones in a structured and meaningful way. Again, there is no information or evidence to suggest that Ratan Thyiam is known for his work in music.

Option 4, social reform, refers to efforts made to bring about positive changes in society and improve the lives of people. While Ratan Thyiam may have contributed to social reform through his work in theatre, there is no specific mention of him being primarily known for his work in this field.

Therefore, the correct answer is option 3, theatre. Ratan Thyiam is recognized for his contributions and achievements in the field of theatre.