Why was "por lo tanto" marked wrong in this test?

Certainly, "por lo tanto" is indeed a correct phrase and is often used to mean 'therefore' in Spanish. However, in the context of this test, it seems that the hint was guiding you towards an alternative that is considered more formal: "con lo cual".

Both phrases can be used to draw a conclusion or show the result of something in a sentence, but "con lo cual" is typically used in more formal writing or speech. It's a subtle distinction and both are grammatically correct, but for the purpose of this test, "con lo cual" was the preferred answer.

It's great to question and seek clarity on these nuances because it helps to deepen your understanding of the language!