What is In Henoch–Schonlein purpura renal involvement is not seen generally if no involvement till ? and explain in detail?
Renal involvement occurs in 20% to 50% of HSP, usually within 4 weeks of presentation, and less commonly precedes the other symptoms.
The onset of abnormalities may follow the onset of other symptoms by weeks (80% within 4 weeks) or even months (97% within 3 months).

Coming back to question

According to me the best answer is 4 months because renal involvement may be seen up to 3 months after the onset of other symptoms (examiner is asking about the renal involvement after onset of disease, not after the onset of other symptoms).
"Renal manifestations usually start at the onset or within 3 months of the onset of other symptoms" — Pediatric nephrology
Other symptoms, e.g. rash, will also take some time to appear after the onset of disease. Thus, renal involvement may occur up to more than 3 months after onset of the disease.