"Gracias por la comida."
Traducción:Merci pour le repas.
Before you can choose between por and para you need to know what aisle of the Spanish supermarket you're on.
These are the only five confusing aisles, the ones that contain both por and para:
- Purpose (para) vs. Reason (por) <= In this case it's talking about the reason to give thanks (with an implied verb: gracias por (preparar) la comida; preparar la comida is your reason)
- Normal recipient (para) vs. Favor recipient (por)
- Moment in time (para) vs. Amount of time (por)
- Destination (para) vs. Route (por)
- Opinion (para) vs. Indifference (por)
If you're not in any of these five aisles (or if you are, but para doesn't fit), use por.
I go deep into the usage of por and para in this post: http://itsnachotime.com/blog/por-para