The sun produces energy from matter in its core through the process of

 The sun generates energy from a process called nuclear fusion. During nuclear fusion, the high pressure and temperature in the sun's core cause nuclei to separate from their electrons. Hydrogen nuclei fuse to form one helium atom.

nuclear fusion

i think it would be b. tissue because an organ system is a group of organs that work together to perform one or more functions. each does a particular job in the body, and is made up of certain tissues.

the second type of galaxy is called a barred spiral, which has a thing called a bar straddling the nucleus