EBill, a nongovernmental organization, launches easybilling.com, a website where all sorts of bills can be paid. It uses specially designed software to allow hassle$-$free transactions in the billing and charges a nominal fee from the users. In this scenario, EBill uses a$($n$)\_\_\_\_\_.$

Question $5$ options:

horizontal applications software

vertical applications software

open source software

copylefted software