try below sql statement, I have change your SQL and also added parameter
string sql = "select pro.ProductID,pro.MenuId,pro.ProductName,pro.ProductDescription,pro.Price,pro.ProductImage from rsa_ProductItemTable As pro join rsa_mastermenu As me on me.MenuId=pro.MenuId where pro.MenuId =@MenuId";
SqlDataAdapter da = new SqlDataAdapter(sql, con);
da.SelectCommand.Parameters.AddWithValue("@MenuId",menuid);
DataSet ds = new DataSet();
da.Fill(ds);