Plasticine is a solid and not a liquid.
A solid is a substance that has a fixed shape and is incompressible. A liquid is also incompressible, however it has no fixed shape and it takes the shape of the container it is in. A gas, in comparison, is compressible and does not have a fixed shape.
Plasticine is a clay like material and is used for modeling. If we use the above mentioned criteria to figure out if Plasticine is solid or liquid, it becomes somewhat confusing. Plasticine is incompressible and has no definite shape. It takes any shape we mold it into and thus can be claimed to be a liquid (as Daniel has done). However, one must remember that in case of a liquid, not a whole lot of effort is required to change the shape of the material, we simply have to change the container. Plasticine is also incompressible. More importantly, it maintains its shape, if left undisturbed. And a lot of effort is required to change the shape of Plasticine. Also, when placed in a container, it does not take the shape of the container on its own.
Hence, we can see that Plasticine is in fact a solid and not a liquid.
Hope this helps.