Pagaba las cuentas del hotel con tarjeta de crédito. el mes pasado anoche de vez en cuando hoy

pagaba las cuentas del hotel con tarjeta de crédito.

Translation:
[blank] I would pay the hotel bills with a credit card.

The options:
El mes pasado - "Last month"
Anoche - "Last night"
De vez en cuando - "Every now and then"
Hoy - "Today"

The sentence above is imperfect tense (it conveys something that used to occur continually but was interrupted). This means the first part of the sentence must be the same tense - it must convey that something used to occur continually. The first, second, and last options convey something that happened in preterite tense (a tense used to speak of events that happened in the past and had a definite end). This means the correct choice is De vez en cuando, as it is the only option in the same tense as the statement.

De vez en cuando pagaba las cuentas del hotel con tarjeta de crédito.

1) el chico joven 2) el señor mayor