What is the correct spelling of the missing word? "By the time the clock chimed for the ______ time, Peter was already up the stairs and in his room."

The correct spelling of the missing word is "twelfth." This spelling is derived from the ordinal form of the number twelve, which indicates the position of something within a sequence. In the English language, the suffix "-th" is commonly used to form ordinal numbers from cardinal numbers, which in this case is twelve.

Using "twelfth" demonstrates the proper formation of the ordinal version, which often includes an "f" before adding the "-th" suffix. This distinguishes it from other forms of the word. The other spellings presented do not conform to standard English spelling rules for ordinal numbers, making "twelfth" the only correct choice in this context.