The relationship in the pair "Grapes - Bunch" is that the second word is the collective noun used to describe a natural grouping of the first. Grapes naturally grow in clusters known as a bunch.
Looking at the options:
Tree - Forest
A forest is composed of many trees. While this is a collective grouping, the term "forest" refers to a large area covered with trees rather than the specific natural grouping of individual items.
Sheep - Herd
A herd is the collective noun for sheep. In the same way that grapes are naturally grouped together in a bunch, sheep are often grouped together in a herd.
Flower - Garden
A garden is an area where flowers are planted, but it's more of a designed space rather than a natural cluster like a bunch of grapes.
Star - Galaxy
A galaxy is a massive collection of stars. Although similar in the sense of being a grouping, galaxies pertain to celestial bodies and are on a completely different scale and context.
Since the relationship in "Grapes - Bunch" is based on a naturally occurring collective noun, the pair that fits best is:
Sheep - Herd
Thus, the correct answer is Option B.