Here's why:

* Structure: Plants have a rigid structure thanks to cell walls made of cellulose. This structure provides support and allows them to stand upright.

* Form: Plants maintain a defined shape and don't readily change their form like liquids or gases.

* Density: Plants have a relatively high density compared to liquids or gases.

However, plants also contain liquids (like water in their cells and sap) and gases (like oxygen produced during photosynthesis). So, while the plant itself is primarily solid, it contains other states of matter within its structure.