Which sentence has a compound subject? a laser printer can receive a fax transmission, and a computer printer can receive one, too. a fax machine has a sensor that reads the paper you are sending. without inventions like telephone lines and satellite transmissions, communication would be much more challenging. telephones, fax machines, and electronic mail transmit messages at many businesses.

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

The subject of a sentence is the noun that is doing the action.  For instance, in the following sentence “boy” is the subject:  The boy kicked the ball.  A sentence, however, can have more than one subject, and this will be indicated usually with the conjunction “and.”  When a sentence has more than one subject performing the same action, this is called a compound subject as in The boy and girl laughed.  With that in mind, we can see that the final sentence of the sentences you provided has three subjects (telephones, fax machines, and electronic mail) that are performing the action “transmit.”

buy a huge library full of all the books ever written

b. b. b. is the anoswer