Answer: option b is correct .His time running this half marathon was abysmal compared to his past performances .
Explanation: The word Abysmal is used as an adjective which means immeasurably low or wretched , extremely poor or bad.
In this question , his present performance is compared to his previous performance in the marathon so the correct word or adjective which can be used is Abysmal.
option a which is , detrimental which means tending to cause harm doesn't bring out the correct meaning and option c i.e. perturb which means make someone anxious and option d repugnant means extremely distateful. Abysmal is used when a comparison has to be drawn.