He was warned at the outset of his career.

Answer: Option B

End : a final part of something, especially a period of time, an activity, or a story.
Beginning : the point in time or space at which something begins.
Middle : the point or position at an equal distance from the sides, edges, or ends of something.
Entrance : an opening, such as a door, passage, or gate, that allows access to a place.
Outset : the start or beginning of something.

Synonym of Outset is Beginning