Final answer:
The correct irregular progressive tense verb is 'están sirviendo' which means 'they are serving'.
Step-by-step explanation:
The correct irregular progressive tense verb in this sentence is están sirviendo. The verb servir (to serve) is conjugated as están sirviendo in the present progressive tense to indicate an ongoing action. In this sentence, it means 'they are serving'.
Learn more about Irregular Progressive Tense Verb in Spanish