The control most effective in preventing the use of the wrong master file is an Internal header label, which provides a unique identification for the file.
The control that could best have prevented a computer operator from using the wrong master file when updating a transaction file, leading to unreadable master file data, is A) Internal header label. An internal header label uniquely identifies a file and includes details about the file's contents, creation date, and other relevant metadata. By checking this label before a file is processed, an operator can ensure that the correct file is being used for the update.
While the other options, like validity check, check digit, and parity check, are data integrity and error-detection controls, they do not specifically address the issue of identifying and confirming file usage. A validity check ensures that data is within a specified range or set of values, a check digit provides an additional number in data entry processes to help confirm that the rest of the data has been entered correctly, and a parity check detects single bit errors in transmitted data or stored data.