Questions: Chicken is required to be cooked to a temperature of at least 165°F (74°C).

The answer is the fourth one (or D): $165^{\circ} \mathrm{F}\left(74^{\circ} \mathrm{C}\right)$.

Explanation for each option:

1. $135^{\circ} \mathrm{F}\left(57^{\circ} \mathrm{C}\right)$: This temperature is too low to ensure that chicken is safe to eat. Cooking chicken to this temperature would not kill harmful bacteria such as Salmonella and Campylobacter.
2. $145^{\circ} \mathrm{F}\left(63^{\circ} \mathrm{C}\right)$: This temperature is also insufficient for chicken. While it might be safe for some other types of meat, it is not high enough to ensure the safety of chicken.
3. $155^{\circ} \mathrm{F}\left(68^{\circ} \mathrm{C}\right)$: This temperature is closer to the safe cooking temperature for chicken but still not adequate to ensure all harmful bacteria are killed.
4. $165^{\circ} \mathrm{F}\left(74^{\circ} \mathrm{C}\right)$: This is the recommended minimum internal temperature for cooking chicken to ensure it is safe to eat. At this temperature, harmful bacteria are effectively killed, making the chicken safe for consumption.

Summary: Chicken should be cooked to an internal temperature of at least $165^{\circ} \mathrm{F}\left(74^{\circ} \mathrm{C}\right)$ to ensure it is safe to eat.