B

Step 1: Identify the regions of the graph. The state of the material transitions from solid to liquid as heat is applied.
Step 2: Recognize that at point O, the material is in solid form.
Step 3: As heat continues to be added, the temperature rises until it reaches the melting point, causing a phase change.
Step 4: Point P represents the point during the transition between the solid and liquid phases (melting).
Therefore, the state of the particle at P is liquid (or in the process of melting).