By default, the <OPTION> tag is treated by Html Agility Pack as "Empty", which means it does not need a closing </OPTION>. In this case, the closing tag is discarded. You can change this behavior using the HtmlNode.ElementFlags collection.

Here is a code that should do what you want:

HtmlDocument doc = new HtmlDocument();
HtmlNode.ElementsFlags.Remove("option");
doc.LoadHtml(yourHtml);

foreach (HtmlNode node in doc.DocumentNode.SelectNodes("//select[@id='onoffaci']//option"))
{
    Console.WriteLine("Value=" + node.Attributes["value"].Value);
    Console.WriteLine("InnerText=" + node.InnerText);
    Console.WriteLine();
}