The product owner is responsible for A. The company usually knows when it would like its product to be available to the public. It is the duty of the other members of the team to figure out how the products can be available within the given time frame. The product owner should not only choose release dates based on what he thinks will be convenient for the company.
It is important for the product owner to recognize how hard or challenging it would be to get the product released in a certain period of time. This may take some research and also advice from people who already have more product releases. The product owner should also collaborate and work with the team not only for motivation but to check the progress of the product that is about to be released.