To access OptionData from a dropdown list, the straightforward approach may be to return the list to the API client and then use Custom serialization to access the text within. 

A more easier approach would be to access the text directly as seen in the example.

api.GetObjectFieldValue<string>("//*[@name='Dropdown']/fn:component('UnityEngine.UI.Dropdown')/@options[1]", "text");

In this example, the index predicate is used to access the individual list item and then the text property.