For a text based option list you can use the `SingleLineTextFieldDefinitionData.List` property as explained in the given answers by @user978511 and @Siva.

If you want to populate the values in the list with Keywords from a Category, you will have to use the `KeywordFieldDefinitionData` class instead for your field. While in the SDL Tridion UI, it is initially shown as a Text field, this is underwater a Keyword field once you select a Category for the list.

The `KeywordFieldDefinitionData.Category` property is where you will be able to set the Category from which you need the Keywords. The `KeywordFieldDefinitionData.List` property should be automatically populated from that point onwards.