In the sentence given, the correct irregular progressive tense verb that fits is "están sirviendo."
Here's why:
1. "Están" is the correct form of the verb "estar" (to be) for the subject "Matilda y Fernando" which is plural, so it should be conjugated in the "ellos/ellas/ustedes" form.
2. "Sirviendo" is the present progressive form of the verb "servir" (to serve) in Spanish. In the present progressive tense, you use the verb "estar" followed by the present participle form of the main verb.
Therefore, the correct option is:
Matilda y Fernando están sirviendo la cena mientras nosotros miramos la montaña.