How does being heterozygous for the sickle cell trait provide resistance to malaria?

Heterozygous individuals have some sickle-shaped cells that resist malaria infection.

Heterozygous individuals have all sickle-shaped cells and are fully resistant to malaria.

Heterozygous individuals have no sickle-shaped cells and are fully susceptible to malaria.

Heterozygous individuals have normal cells that resist malaria infection.