Which of the following has not yet been proven but will add support to the big bang theory if true?

the universe is expanding

overall density of ordinary matter must be roughly 4% of critical density

there is an abundance of the light elements h, he, li

there is a widespread cosmic microwave background radiation

Redshift shows that the universe is expanding, so that is already true.

I don't know anything about B

I'm pretty sure C is already true

And D has already been proven as fact.

I would go with B, but correct me if I'm wrong.

The correct answer is -

Mitochondria produce energy from food, while chloroplasts produce food from the Sun’s energy.

Mitochondria is a specialised membrane bound organelle that is present in the eukaryotes (such as plant and animals) and it is considered as the power house of the cells. It helps in the generation of large amount of energy in the form of ATP (adenosine triphosphate) through cellular respiration (that depicts complete oxidation of food, such as glucose).

Chloroplast is a membrane bound organelle, which is present in the photosynthetic organsisms such as green plants and algae. It is the site for photosynthesis, which is a process of formation of food using sunlight in the presence of carbon dioxide and water.

Thus, second option is the right answer.