Richard Nixon's main purpose in delivering his resignation speech was primarily to announce his resignation from the presidency in response to the Watergate scandal. While elements of defending his innocence and encouraging the public to support President Ford can be considered, the overarching goal was to take responsibility for the political crisis at hand and step down to allow for a peaceful transition of power.
Among your options, the most fitting purpose would be:
To announce his resignation and admit his guilt in order for redemption.
Nixon acknowledges the gravity of the situation and the impact on the nation, indicating a complex interplay of taking responsibility while also attempting to frame his presidency and actions in a broader context of service to the country.