Response body should be in Json

I want to display response body in json   . But i am getting plain response body can any one please help me out. it is urgent.

 [HttpGet]
        [Route("CountUserGroup")]
        public IActionResult CountUserGroup(int id)
        {
            var userData = user.UserCount(id);
            dynamic jObj = new Object();
            jObj = userData;

            return Ok(jObj);

        }