Hola Pratibha

There is more than one test-question using "sería" but I imagine you may be referring to this one:

Yo ___ muy pequeña cuando ellos llegaron al pueblo.

(I was probably very little when they arrived in the village.)

Hint: The speaker is expressing probability about what happened in the past

The literal answer would be "era" as you said: I was (describing the past) but the hint is giving you more information about the specific nuance in this case, which is when you speculate about the past (what the lesson is about), in which case you need the conditional "sería". 

I hope it clarified it.

Saludos