Which one of the following does not remain to be a planet now?

The correct option is Pluto.

In August 2006, the International Astronomical Union (IAU) reclassified Pluto as a "dwarf planet" because it does not meet all three criteria used to define a full-sized planet in our solar system.
To be considered a planet, a celestial body must:
1. Orbit around the Sun.
2. Have sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape.
3. Have cleared the neighborhood around its orbit.

While Pluto satisfies the first two criteria, it has not cleared its neighboring region of other objects in its orbit (the Kuiper belt). Therefore, Pluto is no longer classified as a traditional planet.