1. no. The cost per visit is inversely proportional to the number of visits.
2. yes. The cost is proportional to the number of tickets, hence a linear function of the number of tickets.
3. no. The population is modeled by an exponential function.
4. yes. Distance is proportional to time at a constant speed.
5. yes. (maybe) In the context of this question, "at a constant rate" is presumed to mean "by a constant amount". In that case, growth is linear.
If the rate expressed as a constant percentage (say, 3%) per year, then growth is exponential, not linear.