Option price consist of 2 components.
Option price = time value + intrinsic value.
Time value = amount of time left till expiry
Intrinsic value = difference between spot and strike and some factor of IV (implied volatility)
So given your case, you purchased a 15000CE when the market is at 15800. Let’s say your the price of your option was Rs. 830*75(lot size). So applying the formula,
830 = 20(time value) + 810(intrinsic value)
If the market moves from 15800 to 15900, you’ll get 100 rs more in intrinsic value. As days go by, you’ll lose money in time value. So towards the expiry, you’ll be left only with the intrinsic value. (ie) difference between your ITM option and spot price.
The value of money you loose every day when you hold an option is called theta decay.
So if you end up buying 15000CE when the market is at 15800, and the market remains at 15800, you’ll end up making a loss because of theta decay. (ie) Basically your time value decreases. The market needs to move significantly higher for you to make a decent return.