Define "Delta" in the context of options pricing.

Delta is fundamentally defined as the rate of change of an option's price relative to changes in the price of the underlying asset. This means that delta quantifies how much an option's price is expected to move when there is a $1 change in the price of the underlying asset. A delta value can range from -1 to 1 for puts and calls, respectively, indicating the sensitivity of the option's price to fluctuations in the underlying market price.

Understanding delta is essential for traders and investors because it helps them gauge potential price movements of options in relation to their underlying securities. A positive delta (for call options) suggests that the option's price will rise as the underlying asset's price rises. Conversely, a negative delta (for put options) indicates that the option's price will drop as the underlying asset's price rises.

The other choices relate to various aspects of options trading but do not accurately describe delta. While total value, volatility, and expiration price dealing with options are important, they serve different purposes within the realm of options pricing and do not reflect the specific function of delta. Hence, the definition of delta provided is critical in understanding how options are priced and how they behave in relation to their underlying assets.