Final answer:
The cardinality of the set M, representing all finite strings that can be made from letters in A, is infinite because there's no limit to the length of the words, and the combinations can continue indefinitely.
Step-by-step explanation:
The cardinality of the set M is actually infinite. Considering M consists of all possible finite strings or words that can be formed using the letters in A, we must account for the fact that even though each word is finite, there is no limit to the length of these words, and the process of creating new combinations never ends. For example, if the letter set A contained just two letters, say 'a' and 'b', the set M would include 'a', 'b', 'aa', 'ab', 'ba', 'bb', 'aaa', and so on. The list continues indefinitely, as we can keep adding letters to create longer words. Therefore, even though each individual word is finite, there are an infinite number of such finite words, making the cardinality of M countably infinite. This concept is similar to the way natural numbers are infinite; for every natural number, there's always one more that you can count.