According to Success Criterion 1.3.1 (Info and Relationships), related options in a select box should be wrapped in an optgroup element.
This example should emit a warning about the non-use of option groups. Whether it is appropriate for the select box is a judgement call, based on the options available. For instance, if Option 2a and Option 2b are related to each other, they should be contained within an optgroup element with an appropriate title (eg. "Option 2").