A series is given with one term missing. Choose the correct alternative from the given ones that will complete the series.
?, HI, OP, WX

Let us look at the first letters of each pairs in the series:
Let the missing first letter be x
1
​
. The sequence of first letters is:

x
1
​
,H,O,W
Converting them into their alphabetical numerical positions:

x
1
​
,8,15,23
Let's find the differences between adjacent terms:

From 8 to 15: 8+7=15

From 15 to 23: 15+8=23

Following this increasing difference pattern (+6,+7,+8), the difference before 8 should be +6:

x
1
​
+6=8⟹x
1
​
=2
The 2
nd
letter of the alphabet is B.

Since every element in the series consists of two consecutive letters (HI, OP, WX), the term beginning with B must be BC.

Let's verify with the second letters (y
1
​
,I,P,Y→y
1
​
,9,16,25):

9+7=16

16+9=25
Here the differences are +7,+9. The preceding difference should be +5:

y
1
​
+5=9⟹y
1
​
=4
The 3
rd
letter is C. Thus, the missing term is BC.