Hello @kartik,
You could have your Web Api action return an HttpResponseMessage for which you have full control over the Content. In your case you might use a StringContent and specify the correct content type:
public HttpResponseMessage Get()
{
    return new HttpResponseMessage()
    {
        Content = new StringContent(
            "<strong>test</strong>", 
            Encoding.UTF8, 
            "text/html"
        )
    };
}
or
public IHttpActionResult Get()
{
    return base.ResponseMessage(new HttpResponseMessage()
    {
        Content = new StringContent(
            "<strong>test</strong>", 
            Encoding.UTF8, 
            "text/html"
        )
    });
}
Hope it helps!!
Thank You!!