Select the combination of letters that when sequentially placed in the blanks of the given series will complete the series.

dq_pln_qt_lnd_tpl_dq_p_n

 t, d, p, q, n, t, I

We put all the option in blank area and check who create particular pattern

Option (1) t, t, q, p, n, I, d

dq_pln_qt_lnd_tpl_dq_p_n

After filling the blanks with the letters/numbers in the option given above, we get 

d q t p l n  t q t q l n d p t p l n d q l p d n

No particular pattern is followed here

Option (2) t, p, d, q, l, n, t

da_pIn_qt_Ind_tpl_da-p_n

After filling the blanks with the letters/numbers in the option given above, we get;

d q t p I n p q t d I n d q t p l l  d q n p t n

No particular pattern is followed here also

Option (3) t, d, p, q, n, t, I

da_pIn_qt_Ind_tpl_da_p_n

After filling the blanks with the letters/numbers in the option given above, we get:

d q t p I n d q t p l n d q t p l  n d a t p l n

The pattern is repeating

Thus,  t, d, p, q, n, t, I is correct