Answer:
The verb to indicate that it is an activity that is currently in progress is:
- Ellos no <u>están</u> <u>pagando</u> por el boleto.
Explanation:
The Continuous Tense in <em>Spanish</em> has two verbs, the be or <em>"ser o estar"</em>, and the principal verb finished in <em>"ando"</em> or <em>"endo"</em>, in the case of the verb <em>"pagar"</em> ends in <em>ando</em>, for each Personal Pronoun the forms of <em>"ser o estar"</em> are:
1. Yo estoy pag<u>ando</u>.
2. Tú estás pag<u>ando</u>.
3. Él está pag<u>ando</u>.
4. Ella está pag<u>ando</u>.
5. Ello está pag<u>ando</u>.
6. Nosotros estamos pag<u>ando</u>.
7. Ustedes están pag<u>ando</u>.
8. Ellos están pag<u>ando</u>.
For this reason, the selected form is "están".