Why is loud sound heard during the process of Resonance?

Loudness depends on the amplitude of sound wave . In resonance, amplitude increases when natural frequency matches with the frequency of external force. As amplitude increases, loudness of the sound increases.

A flute has a number of holes. Why?

The sound produced in a flute is due to the vibration of air column in the flute. The frequency of sound produced depends on length. The frequency is inversely proportional to the length of the air column. By closing or opening the holes , the length of the air column is changed and hence the frequency can be changed.

Flash of lightning is seen before the sound of thunder. Why

   Both sound and light energy are released at the same time. In air the speed of light is 300000000 m/s and speed of sound is 340 m/s. Hence the light reaches the observer instantly and sound after some time.

    For example, if an electric discharge  between the clouds takes place at a distance of 1km from the observer then lightning flash is seen instantly but sound of thunder will be heard after about 3 seconds.

Sky appears blue, why?

Question: Why does sky appear blue ?
Answer: Sky appears blue due to the scattering of light by air molecules. Blue components (violet, indigo, blue colours) of white light are scattered the most owing to their short wavelengths compared to other colours. This scattered blue light reaches our eyes and the sky appears blue.