_____ protocol is used for uploading and downloading files between different…

Protocols and standards used in data communication play different specific roles: some govern the transfer of files between computers, some govern the structured exchange of business documents between organizations, some govern how independent networks interconnect at the signaling level, and some are simple control characters that manage the flow of an already-open connection. Identifying the right protocol means matching the function described in the question -- moving files between computers -- to the category whose defining purpose is file transfer.

XMODEM is exactly this kind of protocol: it packages a file into fixed-size blocks, adds a checksum or CRC for error detection, and moves that data between two computers over a serial link or modem connection -- this is precisely uploading and downloading files between different computers.

• X.12 is an ANSI standard for electronic data interchange (EDI) -- it structures business transaction documents such as purchase orders and invoices for exchange between organizations' computer systems, a different task from moving arbitrary files.

• X.75 is an ITU-T signaling standard that governs how separate X.25 packet-switched networks interconnect, operating at the network-to-network signaling level rather than as a file-transfer method.

• XON is one of a pair of software flow-control characters (with XOFF) that resumes data flow on an already-established serial connection, managing transmission flow rather than transferring files.

Answer: XMODEM -- the only offered option whose defining function is transferring files between computers.