The letter “R” was used as an abbreviation for “received” back in the times when messages were send via telegraphy (in Morse code), and the practice of confirming that a transmission was received by sending an “R” back was extended to spoken radio communication at the advent of two-way radio during World War II.
The phonetic alphabet used by the British and American military during the World War II was: Able, Baker, Charlie, Dog, Easy, Fox, George, How, Item, Jig, King, Love, Mike, Nan, Oboe, Peter, Queen, Roger, Sugar, Tare, Uncle, Victor, William, X-ray, Yoke, Zebra
When a soldier or a radio operator said “Roger” after receiving a transmission, he was simply saying “R” for “received”.
The alphabet has changed since then, but the practice of replying to a message by saying “Roger” stuck.
Source:Origin of the phrase “Roger that” in English
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