When constructing a multi-guess test, including one throw-away answer (one that is clearly wrong) is a common tactic.

One can find justifications for this tactic based on allowing the test taker to eliminate one alternative and thus be encouraged…

There are other possible reasons, however…
1st: Doing this improves the statistics of the students. Random guessing on a 4 answer question would mean 1in4 correct answers, but – if students who are guessing can easily eliminate one answer this rises to 1in3 (a gain of ~8.33%).
This seems small, but pushing students over some arbitrary standard often means using every trick at one's disposal.

2nd: [​This is probably more common than is admitted.] Properly, the choices should includ the correct answer and answers that represent common mistakes (ie. 6+9=? would have 15 and 16 with 16 representing a failure to carry the one properly). However, creating 'reasonable' wrong answers is not as easy as it might sound, so making one answer 'out there' is easier.

3rd: Putting clearly wrong answers on a test can help the teacher spot students who give up and just guess. [​If different periods/sections take tests with the answers in different orders,, it canalso be used as an anti-cheat device.]

4th: Test creators sometimes succumb to humor… [​A question in one of my economics clases concerning which type of bond was the safest investment included 'James Bond' as one of the answers.]