The correct answer is B. 1.0 beat per second.
A seconds pendulum is a pendulum that swings from one side to the other in 2 seconds. This means that it completes one cycle of oscillation in 2 seconds. The period of a pendulum is given by the formula:
$T = 2\pi\sqrt{\frac{L}{g}}$
where $L$ is the length of the pendulum and $g$ is the acceleration due to gravity. For a seconds pendulum, $L = 0.9944$ meters.
The frequency of a pendulum is the number of cycles it completes per second. It is given by the formula:
$f = \frac{1}{T}$
For a seconds pendulum, $f = 0.500$ Hz. This means that a seconds pendulum executes 1.0 beat per second.
Option A is incorrect because a seconds pendulum does not execute 0.5 beats per second.
Option C is incorrect because a seconds pendulum does not execute 2.0 beats per second.
Option D is incorrect because a seconds pendulum does not execute 2.5 beats per second.