Which one set of letters when sequentially placed at the gaps in the given letter series shall complete it?
ab_d_aaba_na_badna_badna

The pattern of given letter series is :-
a a b b
Using the same pattern, we will get complete letter series.
a a b b /a a b b/a a b b/ a a b b

The given letter series in below pattern.
a b c a b
Using the same pattern, we will get complete letter series.
a b c a b/ b c a b c/ c a b c a/ a b

The given letter series in below pattern.
a a b b a a b a
After completing the pattern, we will get complete letter series.
a a b b a a b a/ a a b b a a b a

The given letter series in below pattern.
ab c b
After completing the pattern, we will get complete letter series.
ab c b / abc b /ab c b/ a bcb/ a b

As per given question, the letter series is following the below pattern.
a a a b b b
After using the pattern the complete letter series will be as below.
a a a b b b / a a a b b b

As per given question, the letter series is following the below pattern.
a b c
After using the pattern the complete letter series will be as below.
a b c/ c b a/ a b c/ c b a

As per given question, the letter series is following the below pattern.
r t x y s x y z
After using the pattern the complete letter series will be as below.
r t x y s x y z/ r t x y sx y z

The pattern of given letter series is :-
a d c
Using the same pattern, we will get complete letter series.
a d c / a d c / a dc / a d c / ad c

The given letter series in below pattern.
pp q q/qp p p
After completing the pattern, we will get complete letter series.
pp q q /qp p p/ qq q p/qq p q /qq p q

16 x 2 ÷ 25 + 7 - 4
After replacing the sign according to the given question
16 ÷ 2 + 25 - 7 x 4
Apply the rule BODMAS
B - Brackets first
O - Orders (i.e. Powers and Square Roots, etc.)
DM - Division and Multiplication (left-to-right)
AS - Addition and Subtraction (left-to-right)
             = 16 ÷ 2 + 25 - 7 x 4
             = 8 + 25 - 28
             = 25 - 20
             = 5