¿cuál es el mejor complemento para la siguiente oración? los granjeros usaban carretas de bueyes cuando a) comenzaron a construir el tren b) comenzaban a construir el tren c) comienza a construir el tren

The right answer is a) comenzaron a construir el tren

Los granjeros usaban carretas de bueyes cuando comenzaron a construir el tren. (The farmers used bullock carts when they started to build the train)

Let's analyze this answer sentence by sentence:

1. Los granjeros usaban carretas de bueyes 

In this sentence is used the verb usar (regular verb) in the imperfect tense matching he third-person plural. Recall that, in Spanish, the imperfect tense (el imperfecto) is used to describe past habitual actions or to talk about what someone was doing when they were interrupted by something else. In general, the imperfect is used to talk about past actions, conditions, or events that occurred regularly or frequently or that were in progress in a moment in the past. It is also used to tell time, talk about dates, give a person's age, and describe features, conditions, and feelings in the past. 

2. Cuando comenzaron a construir el tren

In this sentence is used the verb comenzar (regular verb) in the preterite tense matching he third-person plural. Recall that, in Spanish, the preterite tense (el pretérito) is used to describe actions completed at a point in the past. So the Spanish preterite is not used to describe regular or continuous actions in the past with no specific beginning or end. In that case, we use the imperfect. Therefore, we use the preterite to talk about completed actions in the past. So, it is used to talk about beginnings and ends, things that occurred on specific dates or days, at specific times or during specific time periods, and events in a sequence.

Des correcto.
las está pagando
porque esta hablando de una persona “el”