Final answer:
A pager is the device among the options listed that allows only one-way transmission, enabling the person to receive messages but not to respond directly through the device.
Step-by-step explanation:
Among the communication forms listed, a pager is the device that allows only one-way transmission. While a landline phone, cell phone, and fax machine all allow for two-way communication, a pager is designed to receive messages only. People carrying pagers are notified of a message or phone number to call, but cannot respond directly through the pager. This technology was quite prevalent before the widespread use of cell phones and is still used in specific industries, such as healthcare, where immediate notification is crucial. However, since it's a one-way transmission, the person receiving the pager alert must find a phone to make a call if they wish to respond.