The correct answer to this open question is the following.
Although there are no options provided, we can say that Kant meant with that quote was "we cannot imagine a world where everyone makes deceitful promises, because the practice of promising presupposes trust and none would trust anyone in a world where everyone makes deceitful promises."
Emmanuel Kant, one of the most important philosophers of our times, considered that people should be very honest to have a good life and never compromise their word in vain because that would generate that other people would never trust this individual. For Kant, people had moral values and they should never try to cheat or manipulate other people. Respect was one of the most appreciated values for him.