le conocí

Hola Ελισάβετ

Your answer is absolutely correct using "lo". 

We will add the option "cuando lo conocí.." to the exercise as soon as possible.

This case, using "le" as a direct object pronoun (instead of lo) with the transitive verb "conocer" is accepted by R.A.E, as long as it refers to a male, which is the case in the text. It's not accepted when referring to a female though, so if we were talking about meeting a lady we need to say:

cuando la conocí

cuando le conocí

I hope this clarified it.

Inma