Final answer:
Wood is the best material to build a boat from the given options because it is less dense than water, sturdy, and can be shaped to displace water efficiently, which provides the necessary buoyancy for the boat to float.
Step-by-step explanation:
When deciding which object should be used to build a boat, understanding the principles of floating and sinking is key. The ability of an object to float is dependent on its density and shape. For instance, a lump of clay when dropped in water will sink due to its high density and compact shape. However, if that same lump is molded into the shape of a boat, it displaces more water, creates a greater buoyant force, and is thus able to float.
In general, materials that are less dense than water and can be shaped to displace a sufficient amount of water are suitable for boat construction. Given the choices, objects such as flower petals or cloth are light but not structurally strong or waterproof, and rocks are far too dense. Thus, wood is the best material from the given options to build a boat as it is sturdy, has a lower density than water, and can be easily shaped to displace water efficiently, demonstrating good buoyancy.