Choice "D" is correct. A Trojan horse is a program that appears to have a useful function but that contains a hidden and unintended function that presents a security risk (appears to be legitimate but performs an illicit activity when it is run).
Choice "c" is incorrect. Redundant verification is not a computer program.
Choice "b" is incorrect. A parallel count is not a computer program.
Choice "a" is incorrect. A web crawler (also known as a web spider or web robot) is a program which browses the web in a methodical, automated manner. Web crawlers are mainly used to create a copy of visited web pages for later processing by a search engine. Web crawlers can also be used for automating maintenance tasks on a web site. Web crawlers can also be used to gather specific types of information from web pages. There is nothing illicit about a web crawler.