algorithms – Search for an element in an array whose index number and element value are identical

Given a separate sorted whole number array A[1,2…..n], the narrowest upper limit to check for the existence of any index I, for which A[I]= I is equal to _______________

I thought I would answer here that the average time complexity would be O (1), because directly get the search index and then check if A[I]= I, but answer with O (log n). Please help me, what will be the correct answer?