Garnish is a culinary term used for describing decorations for food. Garnish is usually made from ingredients that you can eat, and tends to be easily manipulated in shape.
From the available options, it can be said that the produce that is most flexible in shape would be (C) cucumbers. Citrus fruits are generally not flexible, though the peeling is – but you cannot consume the latter. Croutons are also not malleable, and cream puff pastries tend to be used as the main ingredient, not the garnish.