2.

There is

shift in sound

frequency for observers towards

whom the source is approaching.

The question is not well presented.

There is   shift in sound  frequency for observers towards  whom the source is approaching.

The  gap should be filled with an apparent upward

Explanation:

The Doppler effect (also referred to as the Doppler shift) is the difference in frequency of a wave when related to an observer who has a relative movement to the wave source

The Doppler effect can be observed to occur with all types of waves - most notably water waves, sound waves, and light waves

The Doppler effect describes sound as an effect which is produced from a moving source of wave.

According to the Doppler effect;

When the source of waves in which there is an apparent upward shift in frequency for observers towards whom the source is approaching.

the answer is kevlar