The simple Bohr model is not applicable to He⁴ atom because

As we know that Bohr’s model is only applicable for H-atom and hydrogen like atom (e.g. He⁺, Li⁺⁺ etc. basically one electron atom) so when we look carefully to He⁴ then it has one more electron i.e. total 2 electrons so option (c) is correct. Also due to longer distance than nuclear size electrons are not subject to central force so option (d) is also correct and rest options have no meaning to Bohr model, hence they are wrong.

Therefore option (c) & (d) are correct.