Seven people T, U, V, W, X, Y & Z are standing in a single line facing a milk - booth. X is somewhere ahead of Y. There is exactly one person standing between V and Z. W is immediately behind T. Y is behind both U & W. If W & V are fourth and fifth in line respectively, then whish of the following must be true?

The correct option is Y is sixth.

Let us determine the positions of the seven people in the line, from first to seventh, facing the milk-booth:
1^st (front of the line), 2^nd, 3^rd, 4^th, 5^th, 6^th, 7^th (back of the line)

We are given the following initial placements:
- W is 4^th in line.
- V is 5^th in line.

This gives us the initial partial arrangement:
Line: [ _ , _ , _ , W , V , _ , _ ]

Now, let us apply the remaining conditions step-by-step:

Step 1: Place T using W's position
We are told that "W is immediately behind T." This means T is directly in front of W. Since W is in the 4^th position, T must be in the 3^rd position.
Arrangement: [ _ , _ , T , W , V , _ , _ ]

Step 2: Place Z using V's position
We are told that "There is exactly one person standing between V and Z." Since V is in the 5^th position, Z must be either in the 3^rd position or the 7^th position. Since T is already occupying the 3^rd position, Z must occupy the 7^th position.
Arrangement: [ _ , _ , T , W , V , _ , Z ]

Step 3: Place Y using the remaining positions
We are told that "Y is behind both U & W." Since W is 4^th, Y must stand in a position behind the 4^th spot (i.e., 5^th, 6^th, or 7^th).
Looking at the current arrangement, the 5^th position is occupied by V, and the 7^th position is occupied by Z. Therefore, the only available position behind W for Y to stand is the 6^th position.
Arrangement: [ _ , _ , T , W , V , Y , Z ]

Step 4: Check the remaining conditions
The remaining empty positions are 1^st and 2^nd, which must be occupied by U and X in some order. Since U and X will occupy the 1^st and 2^nd positions, the conditions "X is somewhere ahead of Y" and "Y is behind U" are both fully satisfied because Y is in the 6^th position.
Thus, it must be true that Y is sixth.