The abnormal return in an event study is described as the:

daily return on a security on the announcement date minus the risk-free rate of return.

average return on a security for the 7-day period surrounding an announcement minus the average return on the security for the past year.

average return on a security for the 7-day period surrounding an announcement.

daily return on a security minus the daily return on the overall market.

total return earned on a security on the day of an announcement.