An organism that can exist in both oxygen and oxygen-free environments is a(n)

A. Obligate aerobes: Obligate aerobes require oxygen for growth and cannot grow in its absence.

B. Obligate anaerobes: Obligate anaerobes cannot tolerate oxygen and will not grow in its presence.

C. Facultative anaerobes: Facultative anaerobes can use oxygen for aerobic respiration when it is available but can switch to fermentation or anaerobic respiration in oxygen-free conditions, allowing growth in both environments.

D. Aerotolerant anaerobes: Aerotolerant anaerobes do not use oxygen but tolerate its presence and grow equally well with or without it; they do not gain energetic advantage from oxygen.

A. Proteins are permanently denatured: Heat can denature proteins by disrupting noncovalent interactions; however, saying “permanently” is too absolute — some proteins may refold if conditions return to normal.
B. Membranes become too fluid for proper function: Elevated temperatures increase membrane fluidity (phospholipid tails become more disordered), impairing membrane integrity and function. This is true but incomplete alone.
C. Hydrogen bonds within molecules are broken: High temperature disrupts hydrogen bonds (and other weak interactions) that stabilize protein secondary/tertiary structure and nucleic acids; again this is true but incomplete by itself.
D. Hydrogen bonds are broken and proteins are permanently denatured: This combines two effects (bond disruption and denaturation). It’s closer, but “permanently” may be incorrect in some cases where refolding is possible — and it still omits membrane effects.
E. Hydrogen bonds are broken, proteins are denatured, and membranes become too fluid: High temperatures disrupt hydrogen bonding, cause protein denaturation, and increase membrane fluidity, all of which together explain why growth stops above the maximum growth temperature.