Final answer:
At the highest point of its trajectory, a marble launched straight up has an acceleration of 9.8 m/s² downward due to gravity, so the correct answer is B. 9.8 m/s² (downward).
Step-by-step explanation:
When a marble is launched straight up from the ground and reaches its highest point, its velocity is zero because it is the turning point before it starts to fall back down. However, the acceleration due to gravity is still acting on it. At this highest point, just before the marble starts to descend, the acceleration is 9.8 m/s² downward. This acceleration does not change throughout the marble's journey upwards or downwards, as long as we are ignoring air resistance and other possible forces except for gravity. Therefore, the correct answer to the question is B. 9.8 m/s² (downward).