Solution

The correct answer is option 4.

Concept:

A candidate key is a column, or set of columns, in a table that can uniquely identify any database record without referring to any other data

Explanation:

The following functional dependencies,

U → V, VW → X, Y → W, X → U

S = (U, V, W, X, Y, Z)

Here YZ is an independent key. So, for every key, the YZ should their. Hence only option 4 is containing YZ for all keys.
∴ Hence the correct answer is UYZ, VYZ, XYZ.

Options Verify: