In Bohr’s model of the atom, where are the electrons and protons located?

Mechanical waves require a medium to travel. If there is no medium, the mechanical wave doesn't travel. Since, they are waves; they have a wavelength, frequency, speed and also amplitude. The speed depends on the medium and type of the wave. An example for the mechanical waves is sound wave.

2. E = hf, where E is energy, h is plank constant and f is the frequency. Hence, if the frequency is high, then the energy is high. 

v = fλ , where v is the speed, f is the frequency and λ is the wavelength. If the frequency is high, then the wavelength is low.

According to the given choices, gamma rays have the shortest wavelength and highest energy on electromagnetic spectrum.

3. The wavelength of X-rays are smaller the wavelength of infrared.

According to the v=fλ, lower the wavelength, then higher the frequency.  According to the E = hf, if the frequency is high, then the energy is also high. 

Hence, the correct answer is "d"

4. In a longitudinal wave (compression wave) the particles of a matter move parallel. 

There are two types of waves according to the particle movement. They are transverse waves and longitudinal waves. In transverse waves, the particles of the medium move perpendicular to the direction of the wave and longitudinal waves have parallel movement of particles to the direction of wave.

5. In regard to spend of sound, sound travels slowest in gases.

Solids have the highest speed of sound while gases have the lowest. This is because of the particle arrangement of each phase. Solid phase has very tightly packed particle arrangement and due to that the transfer of sound wave in easy than in gases.