The Spanish language uses the imperfect tense to talk about past actions that were repetitive or in progress at a given moment in the past. There are more uses, though.
Option A is not correct because a completed action in the past would require the perfect tense, not the imperfect.
Option C is also not correct because an action happening right now would require the present tense.
Option D cannot be correct because, among other things, the sentence says "Cuando era pequeña" (When she was a kid) so it is not an action that just happened.
Additionaly, by saying that "[...] hacía travesuras" it does not intend to express that she was naughty one time, rather, it conveys that that behavior repeated itself over time.
