Answer:
option-C
Explanation:
we know that
base of logarithm can be negative , zero and 1
so, base of log must be positive and not equal to 1
so, we will verify each options
option-A:

we can see that base=10 ...which is positive and not equal to 1
so, it is log function
this is FALSE
option-B:

we can see that

we can see that base which is positive and not equal to 1
so, it is log function
this is FALSE
option-C:

we can see that base=1 ...which is positive and equal to 1
so, it is not log function
this is TRUE
option-D:

we can see that base which is positive and not equal to 1
so, it is log function
this is FALSE