Time to find the smallest element on a min-heap- one retrieve operation - $\Theta(1)$
Time to find the second smallest element on a min-heap- requires $2^2 - 1 = 3$ check operations to find the second smallest element out of $3$ elements - $\Theta(1)$
Time to find the $7^{th}$ smallest element - requires $O(2^7-1) = O(127)$ check operations to find the seventh smallest element out of $127$ possible ones - $\Theta(1)$
In short if the number of required operations is independent of the input size $n$, then it is always $\Theta(1)$.
(Here, we are doing a level order traversal of the heap and checking the elements)
If we are not allowed to traverse the heap and allowed only default heap-operations, we will be restricted with doing Extract-min $7$ times which would be $O(\log n)$.
Correct Answer: $D$