The correct answer is c) gases.
Gases are unique among the three main states of matter because they do not have a definite shape or volume. Unlike solids, which have a fixed shape, and liquids, which have a definite volume but take the shape of their container, gases will expand to fill the entire space available to them. This means that gas particles are far apart and move freely, leading to a lack of fixed boundaries.
In contrast, solids have tightly packed particles that remain in fixed positions, giving them a stable shape. Liquids also consist of closely packed particles but are able to flow, allowing them to conform to the shape of their container while maintaining a constant volume. Gases, however, are much less dense and spread out, making them capable of filling any container regardless of its shape.