Put each event in order of its occurrence, according to the big bang theory. note that more accurate calculations of the age of each event are still actively being researched, so there is some variability in estimations that you might find on the internet. a few seconds after the big bang ~800 million years after the big bang ~9 billion years after the big bang ~1 billion years after the big bang
a. formation of hydrogen and helium atoms
b. formation of large nebulae into protostars and eventually full blown stars
c. early massive stars exploded producing supernovae and heavier elements
d. formation of our sun and the earth

I think the events are already chronologically arranged correctly. It always have to start with the simplest elements, the hydrogen and helium. In fact, these are the elements that compose the stars. The nuclear fusion of hydrogen atoms to helium is what gives the stars and the sun their energy. So, this is followed by letter b, which is the formation of stars. Then, when these stars explode a stated in c, they produce even heavier elements. As a result, this big bang theory tells us that the universe is expanding because of this 'explosion'. In the end, it led to the formation of the sun, the planets and the universe. Thus, the arrangement is: a, b, c and d.