Q:

Por vs. para vs. a vs. de

How do we translate the below sentence into Spanish?
Meat is the only source of income for them.
I have the following options:
La carne es el único ingreso por ellos.
La carne es el único ingreso para ellos.
La carne es el único ingreso a ellos.
La carne es el único ingreso de ellos.

A:

The most natural sentence in Spanish would actually be "La carne es su única fuente de ingreso".

A:

La carne es el único ingreso para ellos would be the correct one. 
Apparently, google translate would've given you the correct answer too. Para is usually a pretty direct translation of for, although I'm sure you could find some exceptions
Check the answer below for a better translation, and maybe change whether this is the accepted answer: 
"La carne es la única fuente de ingresos para ellos"

