How do the Exchanges decide on Strike prices for Options?

I’ve seen Strike prices changing from 100 strike intervals to 50 recently. What is the basis on which the Exchanges decide to set the Strike prices?

The selection of strike price intervals is up to the discretion of the exchanges. BSE has set a base rule of 3 strike - 1each for ITM/ATM/OTM. 

Following strike parameter is currently applicable for options contracts on all individual securities on NSE

The strike price interval would be:

Underlying Closing Price Strike Price Interval No. of Strikes Provided In the money- At the money- Out of the money No. of additional strikes which may be enabled intraday in either direction
Less than or equal to Rs.50 2.5 5-1-5 5
> Rs.50 to ≤ Rs.100 5 5-1-5 5
> Rs.100 to ≤ Rs.250 10 5-1-5 5
> Rs.250 to ≤ Rs.500 20 5-1-5 5
> Rs.500 to ≤ Rs.1000 20 10-1-10 10
> Rs.1000 50 10-1-10 10

Ref the following link to know more: 


Additionally, for Nifty Index options which is probably the most traded Index options in India, the strike price is set as below:

Source: NCFM's Derivative Market Dealer's Module