Concept:The error is in part (b) because the preposition "after" is needed to show the correct time sequence between being a school friend and becoming a great man.
Explanation:The sentence describes two events in the past: first, he was a school friend; later, he became a great man.
To connect these events in time, we must use the preposition "after" before "becoming".
Without "after", the phrase "but becoming a great man" sounds incomplete and does not clearly indicate the order of events.
The correct phrase should be "but after becoming a great man".
Therefore, part (b) contains the error.
The corrected sentence is: "He was my school friend, but after becoming a great man he has grown proud enough to forget his old friends."
Answer:(b)