The correct answer C) Two seconds

The second pendulum is a pendulum that completes one oscillation in two seconds. The length of a second’s pendulum is approximately 39.17 inches (99.4 cm) at sea level and at a temperature of 59 °F (15 °C) and it will take exactly two seconds to complete one oscillation (back and forth swing).

Additional Information Option Wise

1.  Half a second: A pendulum that completes one oscillation in half a second would have a much shorter length than a second’s pendulum, and it would be swinging at a much faster rate. Such a pendulum would have a length of around 19.6 inches (50 cm) assuming that the amplitude of the pendulum is small and air resistance is negligible.
2.  One second: A pendulum that completes one oscillation in one second would have a length of around 39.17 inches (99.4 cm) at sea level and at a temperature of 59 °F (15 °C) assuming that the amplitude of the pendulum is small and air resistance is negligible.
3.  Two seconds: A pendulum that completes one oscillation in two seconds would have a length of around 78.34 inches (198.8 cm) at sea level and at a temperature of 59 °F (15 °C) assuming that the amplitude of the pendulum is small and air resistance is negligible. It is a correct statement in the context of the question.
4.  None of these: This option is not applicable to this question as it does not provide any specific option for the time taken for one oscillation of a pendulum.

So, therefore the correct answer is C) Two seconds