Which sound would most likely have the lowest pitch?

a bird chirping
a vacuum-cleaner running
a lawn mower running
a dog barking

-- The chirping bird produces the sound with the highest pitch.

-- Next highest is the barking dog.

-- Second to lowest is the vacuum cleaner.

-- The gas-powered lawn mower produces the sound with the lowest pitch.

(This line up is generally usually typically commonly probably true.  There can be some difference if the dog is a very young puppy of a very small breed, squeaking and yipping with a higher pitch than some birds.  And the lawn mower could sound very close to the vacuum cleaner if they're both electric.  But I'm sure the usual typical common birds, dogs, and lawn mowers line up this way.)

A lawnmower running.

Explanation:

if they have a finite resultant

54 degrees incident angle = reflected angle

angle is from normal turned to line of ray