Well (t,s), why this t and s inside the its autocorrelation, it is for one element and it should have only one parameter in the bracket.
a) Autocorrelation function is a function of time t only.
b) Autocorrelation function is a function of lag s only.
c) Autocorrelation function is a function of both time t and lag s.
d) Autocorrelation function is a constant for any input.