I am afraid I could not reply earlier but I was stuck with some priority work. However, I got this working using MouseClick. I did some digging yesterday and found that MouseClick was not working because I have more than 50 items in dropdown and the one which I want to click is not visible in the opened dropdown. I have to scroll through that and then I can click on it.
What I did is that I have search box in the dropdown to search an item. So I searched for required item and then click on it. Here is the code which I used for it.
HtmlInputText SearchTextbox = DropdownDiv.Find.ByAttributes<HtmlInputText>(
IList<HtmlListItem> OptionLabels = DropdownDiv.Find.AllByTagName<HtmlListItem>(
i = 0; i < OptionLabels.Count;i++)
Thanks for your help Cody.