I see 2 perfectly different ways of interpreting this question:

Consider seating arrangement:
A_B_C

where A is the number of seats in each section.
Let a = the number of seats between passengers and the closest aisle.

EXAMPLE:
If A = 5
S and x denote seats
(x is a seat between S and the aisle)
Sxxxx_ = 4
SSxxx_ = 3
SSSxx_ = 2
SSSSx_ = 1
SSSSS_ = 0
Hence a = 4 + 3 + 2 + 1 + 0 = 10 seats total between passengers and the nearest aisle
This can be found using formula A(A-1)/2

There are 2 types of mean averages to consider:
AVERAGE I = a/A + b/B + c/C
AVERAGE II = (a+b+c)/9


Nonetheless, here are solutions for both (and neither seem to match B!)

A+B+C = 9

INTERPRETATION 1:

Using the sum of consecutive integers from 1 to N-1, the total number of arrangements are:
T = A(A-1)/2 + (B^2-1)/4 + C(C-1)/2
T = [2A^2 + B^2 + 2C^2 - 4A - 4C + 3)/4

[Note: B has a different formula as it has 2 aisles to include]

Average = T/9 = [2A^2 + B^2 + 2C^2 4A - 4C + 3)/36
As A = C, then
Average = T/9 = [4A^2 + B^2 -8A + 3)/36

Now test values:
a) 1-7-1 -> {4 + 49 - 8 + 3]/36 = 48/36
b) 2-5-2 -> {16 + 25 - 16 + 3]/36 = 28/36
c) 3-3-3 -> {36 + 9 + - 24 + 3]/36 = 24/36
d) 4-1-4 -> {64 + 1 + - 32 + 3]/36 = 36/36

This gives ANSWER = (C).


INTERPRETATION II:

Here the average (mean) is M = a/A + b/B + c/C
M = (A-1)/2 + (B^2-1)/4B + (C-1)/2
M = (B^2 - 1)/4B + A - 1 (after considering that A = C)

Now test values:
a) 1-7-1 -> M = 84
b) 2-5-2 -> M = 31
c) 3-3-3 -> M = 8
d) 4-1-4 -> M = 3

This gives ANSWER = (D).

Also, to complicate interpretation of the queston even further, are we to include zero seats between the seats and the aisle or not?

Whether or not my solution is correct, I believe the question is not specific enough in its definition.

I am not able to get the question