Find the correct list, which is the reverse (at all levels) of the given list : [[4, 7, 8], 2, 9, [19, 6], 20, 3] (i) 3, 20
(ii) 9, 2
(iii) [6, 19]
(iv) [8, 7, 4]
(i), (iii), (ii) & (iv)
"Reverse at all levels" means two things at once : reverse the order of the top-level elements, and reverse the contents of every nested sub-list as well.
Step 1 — reverse the outer list. Reading the six elements of
[[4, 7, 8], 2, 9, [19, 6], 20, 3]
from the back gives
3, 20, [19, 6], 9, 2, [4, 7, 8]
Step 2 — reverse each sub-list in place. The two nested lists become [6, 19] and [8, 7, 4], so the complete answer is
[3, 20, [6, 19], 9, 2, [8, 7, 4]]
Step 3 — match this against the fragments offered.
| Position | Content | Fragment |
|---|---|---|
| 1st and 2nd | 3, 20 | (i) |
| 3rd | [6, 19] | (iii) |
| 4th and 5th | 9, 2 | (ii) |
| 6th | [8, 7, 4] | (iv) |
The order is therefore (i), (iii), (ii), (iv) — option 3.
The trap is option 4, which simply lists the fragments in their printed order and looks plausible because it begins correctly with (i). What separates it from the right answer is the position of the sub-lists: in the reversed list, [19, 6] was the fourth of six elements, so reversing puts it third — ahead of the 9 and the 2, not after them. Option 4 leaves both sub-lists where the fragment numbering put them rather than where reversal puts them.
The distinction being tested is between a shallow and a deep reverse. A shallow reverse would give [3, 20, [19, 6], 9, 2, [4, 7, 8]], with the inner lists untouched; the phrase "at all levels" requires the operation to recurse into every element that is itself a list. In code the difference is one line — whether the function calls itself on nested elements or merely copies them.
Hence, the correct sequence is (i), (iii), (ii) & (iv).
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