The text states taht delta is usually between 1 and 0 and being long a call delta is positive When else is delta positive or negative? Is it negative short call? Long Put? Positive Short put?
Long call: Positive delta Short call: Negative delta Long put: Negative delta Short put: Positive delta
It is intuitive. Change in option price = option delta * change in price of security Therefore: Option delta = change in option price/change in price of security With the formula, you can easily work out when delta will be negative or positive.