For this sentence " ellos esperaban en la fila." how do i change the verb to indicate that it was in progress at a time in the past using the correct form of estar for ellos esperaban en la fila.

The proper way to change the verb and indicate that the action was in progress is:

Estaban esperando.

- Ellos estaban esperando en la fila.

Explanation:

The translation of the sentence is:

- They were waiting in line.

When is required to mention that an action is or was occurring at a given time, Progressive Tense is used, which in Spanish is characterized because its main verb ends in "ando" or "endo" and use an auxiliary verb that is regularly "estar", as required to use the Past Tense sentence, the verb "estar", must be in the past too, its conjugation taking into account personal pronouns is:

Yo estaba.Tú estabas.Usted estaba.Él estaba.Ella estaba.Ello estaba.Nosotros/Nosotras estábamos.Ustedes estaban.Ellos/Ellas estaban.

How the progressive form of the verb "esperar" is "esperando" and the personal pronoun used in the sentence is "ellos", the right conjugation is:

Ellos estaban esperando en la fila.

Bruno está hablando con su hermana

The answer is c because it the correct pronoun