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. While in the SDL tridion UI, iot is initially shown as a Text field, this is underwater a Keyword field.

The KeywordFieldDefinitionData.Category property is where you will be able to set the Category from which you need the Keywords.

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.