In Linux, the three most common commands that hackers usually attempt to Trojan are:

A.
car, xterm, grep

B.
netstat, ps, top

C.
vmware, sed, less

D.
xterm, ps, nc

Explanation:
The easiest programs to trojan and the smartest ones to trojan are ones commonly run by administrators and users, in this case netstat, ps, and top, for a complete list of commonly trojaned and rootkited software please reference this URL: http://www.usenix.org/publications/login/1999- 9/features/rootkits.html