You ask so many questions,let me have a crack
1.The change in call option prices was because of many factors:
i)Change in the underlying,i.e.,Bank Nifty moves the option prices move too.
ii)Volatility determines the ferocity of the move,the faster the Bank Nifty falls the faster the call option prices fall.
iii)Time left until expiry,the more the time left the more the option would be worth and vice versa.
iv)When major events are about to take place,option premiums are usually higher in anticipation and option prices tend to have an event premium attached to them.
Investopedia - Understanding Option Pricing
2.Stock price volatility and index volatility are not the same.
Go to this link for Option Chain (Equity Derivatives)
and enter the Stock Symbol and click on the strike price,the column you see as IV is the volatility.
3.INDIA VIX Chart
4.As a broad outline,volatility below 20 is considered as low and above 40 is considered very high.
5.VIX is the volatility index based on the NIFTY Index Option prices.
6.When volatility is low option prices tend to be lower,think of it like the calm sea and when volatility is high so are the option prices.