How are frequency and wave period related?

-- The unit of frequency is "per second"  (Hz), which is [reciprocal time].

-- The unit of period is "second", which is [time].

Do you see where this is going ?

'Frequency' and 'period' are reciprocals of each other.

For any wave ...

Period  =  (1) / (frequency) .

Frequency  =  (1) / (period) .

The answer is b.the gaseous state is electrically positive, while the plasma state is electrically negative.