Hello from MrBillDoesMath!
Answer: The third choice
Discussion:
Generally, log (a/b) = log(a) - log(b)
for positive number a and b.
In our case this translates to:
log {8} ( a/2) = log {8} (a) - log{8} (2)
where log {8}, note curly braces not parentheses, mean "base 8
This is the third choice.
Thank you,
MrB