This is a beautiful example of how you can identify an answer
just by looking at the units, and nothing else.
Frequency of a wave is its 'often-ness' ... how often it wiggles,
how many of them per second. The unit is 'per second' or 1/sec .
That's the unit called "Hertz".
What is it the inverse of ? It's the inverse of "second".
Test the choices to find one that has the units of "second":
Velocity (actually speed) . . . meters/second . . . . . No.
Wavelength . . . . . meters . . . . . No.
Amplitude . . . . . volts, or meters, or some pressure . . . . . No.
Velocity (actually speed) . . . . . meters/second . . . . . Still No.
None of these choices has the units of inverse frequency.
The correct answer is "period" ... the length of time each wiggle takes.
It's not even on the list here !
