Answer:
a. Cryptosporidium parvum
Explanation:
Cryptosporidium is a protist parasite that has a spore phase. These spores are called oocysts. In this state the parasite can survive for long periods outside the host and can also resist many common disinfectants. Especially chlorine-based disinfectants. Due to this resistance, water treatment to remove Cryptosporidium in general is based on coagulation followed by filtration or boiling.
