Why Is the ATM Option Most Sensitive to Changes in Implied Volatility?

The ATM option is most sensitive to IV changes because its entire premium is time value, and time value is primarily driven by IV. Deep ITM options are dominated by intrinsic value (which IV does not affect), and deep OTM options have a lower probability of being affected by IV changes.

The ATM option is at the point of maximum uncertainty, making its price most responsive to changes in expected volatility.