Consider each of the answer choices individually. A) $ 43 $ is between $ 40 $ and $ 40+10=50 $. Since $ 3 $ is less than $ 5 $, $ 43 $ rounds down to $ {\bf 40} $. B) $ 23 $ is between $ 20 $ and $ 20+10=30 $, so it cannot round to $ 40 $. C) $ 38 $ is between $ 30 $ and $ 30+10=40 $. Because $ 8 $ is greater than or equal to $ 5 $, $ 38 $ rounds up to $ {\bf 40} $. D) $ 51 $ is between $ 50 $ and $ 50+10=60 $, so it cannot round to $ 40 $. E) $ 45 $ is between $ 40 $ and $ 40+10=50 $. $ 5 $ is greater than or equal to $ 5 $, so $ 45 $ rounds up to $ 50 $. F) $ 35 $ is between $ 30 $ and $ 30+10=40 $. $ 5 $ is greater than or equal to $ 5 $, so $ 35 $ rounds up to $ {\bf 40} $. The answer choices that round up to $ 40 $ are A, C, and F. The final answer is $ \boxed{\text{ACF}} $.