Sorry for n00b question but is there an index like sensex or Nifty that includes the sum total of the entire stock market?

How to lower Margin requirements?

As we know, the margin req is higher during last few days of FnO expiry. My Avble margins went negative 2 days before Oct ’21 expiry and I had to exit few option positions to get back to positive. I’m wondering if something similar happens in the future, can I lower my margin req by entering into an offsetting trade… for eg: If I am short SBIN CE 550… could buying SBIN CE 600 lower my margin req ?

Pls suggest @ShubhS9

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