(D. travel through a vacuum.) All waves need a medium to wave. In a vacuum there is no medium to wave.
1 and 4 are tire.
2 and 3 are not.
Thermonuclear fusion is when matter is heated to a high degree
There's nothing mysterious about it at all. "Frequency" simply means
"often-ness" ... how often or how frequently something happens.
-- The frequency of traditional meals is 3 per day.
-- The frequency of an equinox is 2 per year.
-- The frequency of my sleeping really late is 1 per week.
-- The frequency of my intense desire to sleep late is 30 per month.
etc.
-- The standard unit of frequency in the SI system is "per second".
The special name for that unit is "Hertz". (Hz)