How do you say "we used to play tennis" in spanish?

a: jugabamos al tenis.
b: jugamos al tenis.
c: jugaban al tenis.
d: juegan al tenis.

which word belongs in the blank?

el libro a ana

a: la
b: le
c: lo
d: ella

This sentence stands for the imperfect tense. 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. So Jugábamos is the conjugation of the verb jugar that matches the first person plural (nosotros) in the imperfect.  So we used to play tennis in the past.  

Le is the indirect object pronoun that matches the third person singular. An Indirect Object Pronoun is a person that receives the action of a verb indirectly. Its purpose is to tell you to whom or for whom something is done. So Ana receives the action of the verb to buy (comprar) here, so this action is being performed for her. Finally:

Le compró el libro a Ana