Final answer:
The correct answer is option A. Bacillus species are the most heat-resistant due to their ability to form endospores, which are highly durable structures that allow bacteria to survive extreme conditions for very long periods.
Step-by-step explanation:
The question concerns the resistance of various microorganisms to being killed by heat. Among the options provided, Bacillus species are the most resistant to heat, largely due to their ability to form endospores. These are dormant, tough, and non-reproductive structures produced by some bacteria, which can survive extreme conditions, including high temperatures, for very long periods.
Endospores enable bacteria such as Bacillus anthracis, the causative agent of anthrax, to remain viable for decades and potentially centuries, far outlasting the resistance capabilities of other bacteria and mold spores. This makes Bacillus species particularly difficult to kill and requires special sterilization methods. Consequently, the correct option to the question of which is the most resistant to killing by heat is A. Bacillus species.