What single-celled pathogens are more complex than bacteria and cause diseases like malaria? ?

The correct answer is protozoans.

Explanation:

Protozoans are the single-celled pathogens, which are more composite in comparison to bacteria and results in diseases like malaria. Malaria is the resultant of the protozoan parasite known as Plasmodium. Malaria in human beings is caused by four distinct species of Plasmodium. These are P. falciparum, P. ovale, P. malariae, and P. vivax.  

However, humans in certain cases become infected with the species of Plasmodium, which generally infect animals. The transmission of the malaria parasite takes place by the female Anopheles mosquitoes, which primarily bites between the dusk and dawn.  

multiple allele inheritance is traits controlled by more than 2 alleles (ex: pigeons having 3 alleles of one gene govern their feather color).   so the answer is a.