Time left $1$:$33$:$46$

Deborah was happily working in the IT department of her former employer until she was laid off for no apparent reason.

When she was advised to pick her things up and leave by the end of the day, she decided to install a piece of software in some servers with the intention of accessing them from her home at a later time even if her usual user was blocked.

What kind of software did Deborah installed?

a$.$ A Trojan horse

b$.$ A keylogger

c$.$ A backdoor

d$.$ A rootkit