Which of the following pairs of an acronym and its expansion is incorrectly…

An acronym is a fixed abbreviation, so its expansion is not free to choose: the body that defined the term also fixed the words, and every letter of the acronym must map in order to the exact word used in that standard or vendor specification. Testing an acronym-expansion pair therefore means checking the letters against those officially defined words, and rejecting any plausible-sounding paraphrase that quietly swaps one of them for a near-synonym.

Applying that letter-by-letter test to the four pairs:

1. CRT - Cathode Ray Tube: a vacuum display tube in which an electron gun fires a beam that deflection coils sweep across a phosphor-coated screen. C maps to Cathode, R to Ray and T to Tube, so the pair matches the standard term.

2. MICR - Magnetic Ink Character Recognition: characters printed in iron-oxide ink along a cheque codeline are magnetised and then sensed by a read head. M maps to Magnetic, I to Ink, C to Character and R to Recognition, so the pair matches the standard term.

3. AVI - Audio Video Interleave: the RIFF-based container Microsoft introduced with Video for Windows in 1992, in which the sound and picture chunks are stored interleaved. A maps to Audio, V to Video and I to Interleave, so the pair matches the standard term.

4. CDROM - the pair lists Computer Disk Read Only Memory, but the Philips-Sony Yellow Book of 1985, standardised as ECMA-130 and ISO/IEC 10149, defines CD-ROM as Compact Disc Read-Only Memory. Here C maps to Compact and not to Computer, and D maps to Disc and not to Disk, so two of the five letters point at the wrong words.

Setting the standard expansions beside the ones printed in the pairs makes the single mismatch explicit:

Exactly one row differs, so the incorrectly matched pair is CDROM - Computer Disk Read Only Memory; the standard expansion of CDROM is Compact Disc Read-Only Memory.