The following are voluntary fasting except

Ashura, Qada', Tasu'a, and Sittah Shawwal are all types of voluntary fasting that can be observed by Muslims. However, there is one option among the given choices that does not fall under the category of voluntary fasting.
Ashura is the voluntary fasting observed on the 10th day of Muharram, the first month of the Islamic calendar. It is a recommended fast as it holds significance in Islamic history, such as the day when Prophet Musa (Moses) and the Israelites were saved from Pharaoh. It is not obligatory, but highly recommended.
Qada' refers to making up for any obligatory fast that was missed due to a valid reason, such as illness or travel. In this case, it is not a voluntary fasting, but rather a required act to compensate for the missed fast.
Tasu'a is the voluntary fasting observed on the 9th day of Muharram. It is an optional fast, mostly practiced by Shia Muslims in commemoration of the martyrdom of Imam Hussain, the grandson of Prophet Muhammad, who was killed in the battle of Karbala.
Sittah Shawwal refers to the voluntary fasting observed for six consecutive days in the month of Shawwal, immediately after the conclusion of Ramadan. It is a recommended act and is encouraged as a way to gain more rewards after the compulsory fasting of Ramadan.
Therefore, among the given options, Qada' is the one that does not fall under voluntary fasting. Qada' fasting is a necessary requirement to make up for any missed obligatory fasts, whereas the others are recommended but not obligatory.