If your worksheet key has a specific code like "RARL," that might be an answer code or typo — but the joke above is the standard one for that riddle.
The likely answer (the "punchline" or worksheet key) is: Or a shorter version: "I'm not a ruler anymore." (The pun is on ruler as a measuring stick vs. ruler as one who governs or controls. The teenage yardstick is rejecting being a "ruler" in the parent-child sense.) If your worksheet key has a specific code
It looks like you're referencing a riddle or a pun from a worksheet (possibly from a math or measurement-themed activity). The phrase "What Did The Teenage Yardstick Say To Its Parents?" is a play on words involving and teenage rebellion/independence . If your worksheet key has a specific code