An alga which can be possibly used in space flight (space alga) is
A. Nostoc
B. Chlorella
C. Ulothrix
D. Spirogyra

Hint: The flight which involves an astronautic application by using spacecraft is called space flight. Space flight includes both human or nonhuman involvement. During the exploration of space by involving human presence, a type of algae is used. This alga is known as space alga.

Complete answer:
> Nostoc is a type of cyanobacteria. The bacteria which are capable of photosynthesis are known as cyanobacteria. Nostoc belongs to the family Nostocaceae. The habitats of the Nostoc are soil, moist areas like rock, the bottom layer of the springs, and lakes. Examples: Nostoc willei, Nostoc boride, etc.
> Chlorella is known as space alga. It is used for the consumption of carbon dioxide provided by the astronauts and the production of oxygen in the spacecraft. The features which make the Chlorella, the space alga are given below:
i) Chlorella can be grown very easily.
ii) It is a very good source of food which is rich in protein, nutrient.
iii) Chlorella comprises a high amount of iron and vitamins.
> Ulothrix is a type of algae. Ulothrix belongs to the family Ulotrichaceae. It possesses filaments with no branches. Ulothrix shows vegetative reproduction. Examples: Ulothrix zonata
> Spirogyra is a green alga, from the family Zygnemataceae. It is mainly found in mineral enriched water. Spirogyra comprises filaments. Both asexual and sexual reproduction is found in Spirogyra.

Note:
Chlorella belongs to the family Chlorellaceae. It does not comprise any flagella. The diameter of the Chlorella varies between, 2-10 $\mu m$. Chlorella is considered a superfood. Chlorella can produce oil under specific circumstances that involve a high amount of polyunsaturated fat and omega-3 fatty acid.