It involves changing data before , or as it is entered into the computer or in other words , it refers to the alteration of the existing data.
The other answers are incorrect because :
Salami techniques : A salami attack is the one in which an attacker commits several small crimes with the hope that the overall larger crime will go unnoticed.
Trojan horses: A Trojan Horse is a program that is disguised as another program. Viruses:A Virus is a small application , or a string of code , that infects applications.
Reference: Shon Harris , AIO v3
Chapter -11: Application and System Development, Page : 875-880 Chapter -10: Law, Investigation and Ethics , Page : 758-759