Final answer:
For a single guess at a shift cipher key, shift the ciphertext so that the most common letter corresponds to 'E', which is the most frequent letter in English. However, the reference to the Zimmerman Telegram and a transposition cipher seems out of context since it implies rearranging letters, not shifting them.
Step-by-step explanation:
If you are presented with a ciphertext that you know is the result of a shift cipher, and you have only one opportunity to guess the key, the best guess would be to use the key that shifts the letters in the cipher by the most common offset used in standard shift ciphers. In a standard shift cipher, also known as a Caesar cipher, a common and simple key is a shift of 3, whereby each letter in the plaintext is shifted three places down the alphabet. However, if the ciphertext is in English, the best single attempt based on frequency analysis could be to shift the letters so that the most common letter in the ciphertext becomes 'E', which is the most common letter in the English language. This approach relies on statistical analysis and may give you the best chance to decrypt the message.
However, it's important to understand that the question mentions a transposition cipher, not a shift cipher, when describing the Zimmerman Telegram. In this context, mentioning the shift cipher might be a typo or confusion, as a transposition cipher does not involve shifting letters but rather rearranging them. But assuming the ciphertext is indeed encrypted with a shift cipher, guessing the shift value that aligns with the most common letter in English as 'E' is likely the best single-key strategy.