Which of these would form first during the development of a complex multicellular organism, such as an animal? a) cell b) organ c) tissue d) organ system

The correct answer is option A, cell

reason -

For being a  multi cellular organism, the organism first need to have multiple cells and thus the first stage of development shall comprise of multiple cell production. In the next stage several multiple cells combine to form a tissue which further reorient  to transform themselves into an  organ. A group of organs is referred to as an organ system and further group of organ systems leads to formation of an animal /species

if you listed your answers right then i believe your answer is b.