Answer: A). Fusion
In star fusions of atoms heavier than hydrogen like helium takes place in the core of the star, at the core the temperature and density of gases is the greatest . It also depends upon the surface area of the star the larger the star the more surface area it will have and larger core will facilitate more profound fusion of gases, this will result in production of heavier elements. The fusion produces heavier element than parent elements. This will form more massive star which results in hot core which can produce heavier elements by fusion. Energy is released at each fusion stage. When the stars becomes massive they exhibit changes like their cores contract and limit in size and temperature increases, such stars are called red giants. Red giants have mass three times greater than the sun. Oxygen is the atom produce in these red giants after fusion of helium and carbon. If the mass of the star becomes greater than sun after fusion, it results in formation of heavier element like iron. When the core becomes iron in a star it explodes and forms supernova. This explosion creates nuclei which are heavier than iron.