How can we make sure that web api returns json data only?

https://www.c-sharpcorner.com/blogs/return-data-in-json-format-from-web-api

Open WebApiConfig.cs file in solution and add mentioned line 

public static void Register(HttpConfiguration config)   
{  
    config.Routes.MapHttpRoute(name: "DefaultApi", routeTemplate: "api/{controller}/{id}", defaults: new   
    {  
        id = RouteParameter.Optional  
    });  
    //To produce JSON format add this line of code  
    config.Formatters.JsonFormatter.SupportedMediaTypes.Add(newMediaTypeHeaderValue("text/html"));  
}