Of course, but students have to answer questions based on how the questions are actually worded. If we go down the route of second-guessing what the question-setter really meant, we open up a can of worms.
Two possible interpretations here:
Pets on board public transport are an increasingly common sight in many cities.
This could be interpreted to mean that there are more pets on public transport.
The sight of pets on board public transport is increasingly common in many cities.
This could be interpreted to mean that a fixed number of pets are being increasingly spotted – perhaps because they are no longer hidden in the last carriage of a train or at the back of the bus.