You must choose the 15-year exemption, the retirement exemption and the rollover for those concessions to apply. However, the 50% active asset reduction applies automatically if the basic conditions are satisfied and you have not specifically chosen that it not apply.
Generally, a choice available under the CGT law must be made by the day you lodge your income tax return for the income year in which the relevant CGT event happened, or within such further time as the Commissioner allows.
The way you prepare your income tax return is generally sufficient evidence of the choice you have made. However, the retirement exemption requires you to keep a written record of the amount you choose to disregard.