Answer: The correct answer is-
paying bills online instead of sending paper through the mail.
Act of reducing can be described as a behavior/ an act that tends to reduce or decline the usage/ amount of particular thing.
Out of the given options, paying of bills through online mode instead of sending papers through mail best describes the act of reducing.
For instance, a recruitment agency such as CSIR ( Council of scientific and industrial Research) ask for online applications instead of sending them hard copy.
This will reduce the amount of paper as all the applicants will apply through online mode.
Thus, option first is the right answer.