The correct answer is A: The number of words or word equivalents in the text portion of the message. In a radiogram header, the 'check' is the word count - the number of words (or word equivalents) in the text portion of the message. This helps verify that the message was received correctly. The check is important for message accuracy. When you receive a message, you count the words in the text. If your count matches the check number in the preamble, it's a good indication the message was copied correctly. If the counts don't match, there may have been an error in transmission or reception. Word equivalents include groups of numbers, mixed letters and numbers, and other standard counting rules used in formal message handling.