Let us go over all of these options.
1. m equals e over c:
e = mc, this means that m = e/c - the first option is correct
2. c equals e over m:
e = mc, this means that c = e/m - the second option is correct
3. e = cm:
e = mc, which is the same as e = cm - the third option is correct
This means that the fourth option, m equals c over e is incorrect, as we already saw that m = e/c, and not the other way round.
