Find the term which doesn't fit into the series given below:

H4Q, K10N, N20K, Q43H, T90E

The series H4Q, K10N, N20K, Q43H, T90E follows three consistent patterns:

$\bullet$ First letters: H($8$th) $\to$ K($11$th) $\to$ N($14$th) $\to$ Q($17$th) $\to$ T($20$th), increasing by $+3$ positions each time.

$\bullet$ Last letters: Q($17$th) $\to$ N($14$th) $\to$ K($11$th) $\to$ H($8$th) $\to$ E($5$th), decreasing by $-3$ positions each time.

$\bullet$ Numbers: Follow the recursive rule $a_n = 2 \times a_{n-1} + (n-1)$, where $n$ is the term position:

$\quad a_1 = 4$

$\quad a_2 = 2 \times 4 + 1 = 9$ (but given as $10$, mismatch)

$\quad a_3 = 2 \times 9 + 2 = 20$

$\quad a_4 = 2 \times 20 + 3 = 43$

$\quad a_5 = 2 \times 43 + 4 = 90$.

Why K$10$N?

K10N breaks the number pattern (should be $9$ instead of $10$), while all other components fit perfectly across the series. This matches option $2$ in the provided choices.