For a given substance at a specified temperature, activity is __________ to fugacity.

For a given substance the activity is defined as ratio of fugacity at conditions of interest to fugacity at standard state, the standard state can be anything where the fugacity is known know, if you keep the standard state same and change the conditions of interest where you want to find out the fugacity then we can say that the activity becomes directly proportional to fugacity.
$$ \Rightarrow {a_i} = \frac{{{f_i}}}{{f_i^o}}$$