Both. If the chair has no arms - you sit ON it. If the chair has arms - you sit IN it (because it, sort of, encloses you).

On. It refers to where you physically are. When you are using a chair your body is physically on the chair and not inside the chair. "I am sitting in the cinema, on a chair." - Your body is physically in the cinema and also on a chair.

sit on a chair sit in an armchair

Dictionary khodeto check kon .Javabe Eliot va British Colombish doestareh .

سلام .این یکی که ساده بود اقا .من همه ی سووالات تو رو می خونم ولی به نظرم رسید این ساده هست و تو اینو بلدی :))))))))))) موفق باشی:)))))))))