Answer:
a. fluid
Step-by-step explanation:
A fluid is any body that has the property to flow, and lacks rigidity and elasticity, and consequently yields immediately to any force tending to alter its shape and thus adopting the shape of its container. Fluids can be liquids or gases according to the different intensity of the cohesion forces existing between their molecules. In liquids, intermolecular forces allow particles to move freely, although they maintain latent bonds that cause substances in this state to have a constant or fixed volume. Gases, by contrast, consist of well-separated moving particles that collide with each other and try to disperse, such that the gases have no defined shape or volume.