Explanation:

Let’s break it down:

- The car has 6 seats: Front row (F1, F2, F3), Back row (B1, B2, B3).

- Seats fill from back row, right to left: B1 (back right), B2 (back middle), B3 (back left), then front row, right to left.

- Y boards first, so Y gets B1 (back right).

- Next two people get B2 and B3, then front seats.

- Z boards second last (so 5th), U is last (6th).

- Z sits in front of V: that means if V is back row, Z is exactly in front (same seat position, row ahead).

- W is to the left of V: same row, just to the left side.

- U (6th) gets F3 (front left).

Now, if you map it out seat by seat from boarding order:

1st: Y - B1

2nd: ? - B2

3rd: ? - B3

4th: ? - F1

5th: Z - F2

6th: U - F3

Because Z is 5th and sits in front of V, V must be in B2 (back middle, boarded 2nd or 3rd). W is to the left of V—in back row, that is B3, so W boards just after or before V.

So, X can't be 4th if 4th is someone who sits in F1. But let's check:

If X is 2nd, boards after Y, takes B2.

If V is B2, X is V; if not, X is 3rd and boards B3.

Let’s fill:

1. Y (B1)

2. V (B2)

3. W (B3)

4. X (F1)

5. Z (F2)

6. U (F3)

Z (F2) is in front of V (B2). W (B3) is to left of V (B2). X’s seat is F1, so 4th to board.

So, here’s the answer:

- Option 4: 4th