The correct words for the blanks are (i) heyday and (ii) precipitous.

Breakdown of the Blanks
- Blank (i): C. heyday

The sentence describes Mike Tyson as "the most feared fighter in the world" during a certain period. The word that best describes the peak of a person's success or power is heyday. This fits perfectly with the idea of him being at the absolute top of his career. "Tenure" refers to a period of time in office or a position, which is too general. "Debasement" is the opposite, referring to a decline in quality or value.
- Blank (ii): F. precipitous

The second part of the sentence contrasts his eminence with his fall. The word "astonishing" suggests the fall was dramatic and sudden. Precipitous means very steep or sudden. This accurately describes a rapid and unexpected decline from a position of power. "Seemly" means polite or decent, which does not fit the context. "Unsustainable" means not able to be maintained at a certain level, which, while related, is not as precise or powerful as "precipitous" in describing the suddenness of his fall.

The Complete Sentence
"Mike Tyson, during his heyday, was the most feared fighter in the world; his fall from eminence was as astonishing as it was precipitous."