You can change the return type to a
Dictionary<IWebElement, string>
Once you have the compiled the list of IWebElements, then you can simply add the associated string with the IWebElement into a Dictionary to return - whoever calls your method will have both the IWebElement and the string for processing.
var options = dropMenu.Options as List<IWebElement>;
if (options != null)
{
var values = options.ToDictionary(option => option, option => option.Text);
return values;
}