Step 1: Observe the pattern in the first row.
The first row has letters: \(N, U, F, (i)\) and numbers: \(21, 14, 9, 6\).
Each number represents the position of the corresponding letter in the English alphabet:
- \(N = 14\), but shown as \(21\).
- \(U = 21\), but shown as \(14\).
- \(F = 6\), shown as \(9\).
- So, \((i)\) must be a letter whose position is \(6\) shown as \(?\).
From the sequence, we notice they are swapping the alphabet position with another in the same row. \(N(14)\) - \(U(21)\). Similarly, \(F(6)\) - ?(9).
So the missing letter is the one with position \(9\), which is \(\boxed{I}\).
Thus, \((i) = I\).
Step 2: Analyze the second row letters.
We have \(H, L, (ii), O\) and numbers \(12, (iv), 15, (iii)\).
Check mapping:
- \(H = 8\), but the number below is \(12\).
- \(L = 12\), number missing \((iv)\).
- \(O = 15\), number below is \((iii)\).
So the rule seems: swap alphabet positions in pairs again.
- For \(H(8)\), the number \(12\) belongs to \(L\).
- For \(L(12)\), the number should be \(8\) (belongs to \(H\)).
So \((iv) = 8\).
Similarly, for \(O(15)\), the number should swap with the missing letter \((ii)\).
Thus, \((ii)\) must be the letter with alphabet position \(10\), i.e., \(\boxed{J}\).
And so, \((iii) = 15\) swaps with \(J\)'s actual position number → \(10\).
Step 3: Final values.
\[
(i) = I, (ii) = J, (iii) = 10, (iv) = 8
\]
Final Answer:
\[
\boxed{I, J, 10, \text{ and } 8}
\]