A is corrent because a validity check determines whether a character is a legitimate item of the given character set. Thus, the validity check ascertains whether a given character is within the desired group (e.g. a field indicating sex of an individual where F=female and M=male would not accept an "A" being coded).
B is incorrect because an echo check is a hardware control wherein data is transmitted back to its source and compared to the original data to verify the transmission correctness.
C is incorrect because a parity check involves a special bit which is added to each character stored in memory and detects if the hardware loses a bit during the internal movement of the character.
D is incorrect because a limit or reasonableness check is a programmed control based on specified limits. For example, a calendar month cannot be numbered higher than 12, or a week cannot have more than 168 hours.
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