Ovidiu's response is, as usual, very helpful and entirely accurate. Being a non-native speaker, here are some additional things that it took a moment for me to wrap my mind around: the use of the definite article here is different from what we would expect in English, and is in some ways a bit of an exception in Romanian too. Generally in Romanian nouns after (aka governed by) a preposition are in their 'nearticulat' (without-an-article) form, but means of transport after 'cu' are one of the exceptions. "el a venit cu taxiul" = "he came by taxi" and probably not "he came with THE taxi", just as Ovidiu explained. So, two important things, both about the definite article on "autobuzul": one, generally speaking in Romanian one does NOT see a definite article after a preposition except in a handful of exceptions, this being one. two, contrary to what it sounds like, this use of the definite article is NOT defining a specific bus ("THE bus"), as a word-for-word (motamo) translation would wrongly imply.

Exactly. Very good points Ovidiu and Paul.

I filled in "by bus" and it is accepted now. For the formula "with the bus", I imagine he is comming with a toy-bus in his pocket...