Select the correct option that will fill in the blank and complete the series.

cab, gef, ljk, rpq, ..........

The correct option is ywx.

To find the missing term, let us analyze the pattern of the given series: cab, gef, ljk, rpq, ...
We can determine the next term by observing the alphabetical position transitions of the first, second, and third letters of each term separately.

1. First Letters of each term:
The first letters of the terms are c, g, l, and r.
Let's convert these letters to their numerical positions in the English alphabet where A = 1, B = 2, and so on:
c = 3
g = 7
l = 12
r = 18
Now, observe the differences between consecutive terms:

$3+4=7$

$7+5=12$

$12+6=18$

Following this increasing difference sequence (+4, +5, +6), the next transition must be +7:

$18+7=25$

The 25th letter of the alphabet is y.

2. Second Letters of each term:
The second letters of the terms are a, e, j, and p.
Converting to numerical positions:
a = 1
e = 5
j = 10
p = 16
Observing the differences between consecutive terms:

$1+4=5$

$5+5=10$

$10+6=16$

Following the same pattern, the next transition is +7:

$16+7=23$

The 23rd letter of the alphabet is w.

3. Third Letters of each term:
The third letters of the terms are b, f, k, and q.
Converting to numerical positions:
b = 2
f = 6
k = 11
q = 17
Observing the differences between consecutive terms:

$2+4=6$

$6+5=11$

$11+6=17$

Following the same pattern, the next transition is +7:

$17+7=24$

The 24th letter of the alphabet is x.

Combining the letters obtained for each position, the next term in the series is ywx.