A pen register is a:

A.
Device that records the caller-ID of incoming calls

B.
Device that records the URLs accessed by an individual

C.
Device that identifies the cell in which a mobile phone is operating

D.
Device that records all the numbers dialed from a specific telephone
line

Explanation:
(Electronic Privacy Information Center, Approvals for Federal
Pen Registers and Trap and Trace Devices 1987-1998, www.epic.
org). Gathering information as to which numbers are dialed from a
specific telephone line is less costly and time-consuming than
installing a wiretap and recording the information.
* There is also equipment that can record the information listed in answers “Device that identifies
the cell in which a mobile phone is operating” and “Device that records the URLs accessed by an
individual”.
* The device referred to in answer “Device that records the caller-ID of incoming calls” is called a
trap-and-trace device.
All of the answers in this question are a subset of the category of traffic
analysis wherein patterns and frequency associated with communications
are studied instead of the content of the communications.