Which of the following celestial bodies is classified by astronomers as a dwarf planet?

Introduction / Context:
In planetary science, celestial bodies in our solar system are classified into categories such as planets, dwarf planets, moons, and small solar system objects. Dwarf planets are bodies that orbit the Sun and are massive enough to be nearly round but have not cleared their orbital neighbourhood. This question asks which of the listed bodies falls into the dwarf planet category.

Given Data / Assumptions:
- The task is to identify a dwarf planet from the options given.
- The options are Neptune, Titan, Eris, and Hydra.
- We assume familiarity with basic solar system classification and the names of planets and moons.

Concept / Approach:
Neptune is a full sized planet, Titan is a moon of Saturn, and Hydra is a moon of the dwarf planet Pluto. Eris, on the other hand, is itself a dwarf planet located in the outer solar system beyond Pluto in the scattered disc region. Knowing these identities allows us to pick Eris as the dwarf planet among the choices.

Step-by-Step Solution:
Step 1: Identify Neptune as one of the eight recognised planets of the solar system. Step 2: Recall that Titan is the largest moon of Saturn, noted for its thick atmosphere. Step 3: Remember that Hydra is a small moon orbiting around the dwarf planet Pluto. Step 4: Recognise Eris as a trans Neptunian object classified as a dwarf planet by the International Astronomical Union. Step 5: Select Eris as the correct answer because it is the only dwarf planet listed.

Verification / Alternative Check:
Astronomy references list five officially recognised dwarf planets in the solar system: Ceres, Pluto, Haumea, Makemake, and Eris. The name Eris appears consistently on this list. Neptune is always grouped with the eight planets, and Titan and Hydra are always listed as natural satellites. Cross checking with these standard lists confirms that Eris is the correct choice.

Why Other Options Are Wrong:
Neptune: A gas giant planet, not a dwarf planet, and one of the eight main planets orbiting the Sun.
Titan: A moon of Saturn, famous for its atmosphere and lakes of liquid hydrocarbons, but not classified as a dwarf planet.
Hydra: A small moon orbiting Pluto, also not a dwarf planet.

Common Pitfalls:
A possible confusion arises because both Pluto and Eris are dwarf planets, and Hydra is a moon of Pluto. Learners may misremember Hydra as a separate dwarf planet or think Titan is one because of its size. Another mistake is to misinterpret the question and pick Neptune simply because it is a familiar large object. Familiarity with the official categories defined by the International Astronomical Union is important.

Final Answer:
Among the bodies listed, Eris is the one that is classified as a dwarf planet.