The answer is D.
Mechanical waves need a medium to travel, for example, sound is a mechanical wave. You are able to hear other people because the sound wave is travelling trough a fluid -air-. If you were to be in space -vacuum- you would not be able to hear anything.
The most well known example of an electromagnetic wave is light. As you may have noticed, we're able to get the light from the Sun to Earth; clearly light -electromagnetic wave- is able to travel in vacuum/space (i.e., without a medium).
Answer:
The required frequency = 0.442 Hz
Explanation:
Frequency 
where;

Then;

However;
and;
mass 


where;

Then;

replacing the values;


C is the answer to the question
Answer:
A. 2.82 eV
B. 439nm
C. 59.5 angstroms
Explanation:
A. To calculate the energy of the photon emitted you use the following formula:
(1)
n1: final state = 5
n2: initial state = 2
Where the energy is electron volts. You replace the values of n1 and n2 in the equation (1):

B. The energy of the emitted photon is given by the following formula:
(2)
h: Planck's constant = 6.62*10^{-34} kgm^2/s
c: speed of light = 3*10^8 m/s
λ: wavelength of the photon
You first convert the energy from eV to J:

Next, you use the equation (2) and solve for λ:

C. The radius of the orbit is given by:
(3)
where ao is the Bohr's radius = 2.380 Angstroms
You use the equation (3) with n=5:

hence, the radius of the atom in its 5-th state is 59.5 anstrongs