Which sentence has a compound subject?

a fax machine has a sensor that reads the paper you are sending.

a laser printer can receive a fax transmission, and a computer printer can receive one, too.

telephones, fax machines, and electronic mail transmit messages at many businesses.

without inventions like telephone lines and satellite transmissions, communication would be much more challenging

The sentence that has a compound subject is the following one: telephones, fax machines, and electronic mail transmit messages at many businesses.

Explanation:

A compound subject is a subject made up of two or more simple subjects that are joined by a coordinating conjunction and share the same predicate. In the sentence above, the compound subject is the coordinated NP "telephones, fax machines, and electronic mail". The predicate this compound subject shares is the VP "transmit messages at many businesses". What is more, the NPs have been linked with the coordinating conjuncction and.