There are no options given for the question asked, but in any case the correct use of the preposition in this sentence is in a sense of "consequence."
<u>Juan feels bad in consequence of of his mistake.
</u>
A synonym in English of the preposition <em>por </em>is "because of".
Prepositions are used to be able to express sentences correctly and give them a more complete meaning, giving them a sense of relationship of dependence between one word and another.
