From gulliver’s travels by jonathan swift of so little weight are the greatest services to princes when put into the balance with a refusal to gratify their passions. what is the main idea of gulliver’s statement? a. serving your rulers’ passions will bring great monetary reward. b. rewarding princes is the best way to balance their power. c. service to princes is the only way to gain respect in life. d. prior service to rulers means nothing once you them.

The correct option is D.
What Gulliver is trying to say in the passage given above is that, no matter how faithfully and loyally you have served the princes before, once you do something that they did not like all the services you have rendered  to them in the past will not even be remembered by them again. 
When you say 'something is of little weight' as used in the sentence above, that mean, that thing have no importance, it is something that can be easily be disregarded.