Explanation:
a strange way to write the problem.
nobody puts the function in brackets. only the input argument.
so, it should be
log(5)
and yes, the actual convention is that if the base of the logarithm is missing, the base is 10 by default.
for the base e the convention is to write "ln" instead of "log" and also skip the base.
so, formally that is
log10(5)
but careful, some modern day geniuses creating programming languages and their functional add-on libraries have started to only provide "ln" and named that command "log" for whatever reasons (I don't know why).
so, with that background the right answer could be
loge(5).
only you know where that question is coming from.
FYI - background : we can convert the logarithm of any base immediately into a logarithm of another base :
loga(b) = logc(b)/logc(a)
and therefore, ultimately, we need only one logarithm, true. but 10 is an easy natural number used in all our normal calculations as every number consists of a combination of various powers of 10.
and while e is involved in a lot of natural processes, it is by itself an "ugly" (irrational) number. having to use it twice in one calculation squares the uncertainty of having to round it at one point.