(b) 'Z' only
Only flower Z would develop into a fruit, as fruit formation requires fertilisation, which occurs only when pollen reaches the pistil (stigma). Flower Z, with both functional stamens and pistil, allows pollination and fertilisation, after which the ovary develops into a fruit.
Since the diagram is not visible, the standard reasoning is: a fruit develops only from a flower that has a pistil (ovary) and has undergone fertilisation. Flowers lacking a pistil (male/unisexual) cannot form fruits. The correct option (b) Z only is the given answer, implying only flower Z has a functional pistil with the conditions for fertilisation. Remember: ovary → fruit after fertilisation (Source: Chapter 7, Section 7.3.2).