The precipitous is considered as an unexpected or abrupt change. This change is considered as a good thing.
The other words such as “difficult” (hard to follow), gentle (smooth nature) and unanticipated nature is not applicable to above context.
Thus, the options (a), (d) and (c) are incorrect.
The precipitous is considered as an unexpected or abrupt change. This change is considered as a good thing.
Therefore, the correct option is – b. Abrupt.