In group `IA` of alkali metals, the ionisation potential decrease down the group,Therefore ,lithium is a poor reducing agent.

Explanation

### Step-by-Step Solution:

1. **Understanding Ionization Potential**:
The ionization potential (or ionization energy) is the energy required to remove an electron from an atom in the gaseous state. In the alkali metals group (Group IA), the ionization potential decreases as we move down the group from lithium (Li) to cesium (Cs).

2. **Ionization Energies of Alkali Metals**:
The ionization energies for the alkali metals are as follows:
- Lithium (Li): 520 kJ/mol
- Sodium (Na): 496 kJ/mol
- Potassium (K): 419 kJ/mol
- Rubidium (Rb): 403 kJ/mol
- Cesium (Cs): 374 kJ/mol

As we can see, lithium has the highest ionization energy among these metals, making it more difficult to remove an electron from lithium compared to the others.

3. **Reducing Agent Definition**:
A reducing agent is a substance that donates electrons to another substance and is itself oxidized in the process. For a good reducing agent, it should easily lose electrons.

4. **Lithium as a Poor Reducing Agent**:
Due to its high ionization energy, lithium does not easily lose electrons in the gaseous state. Therefore, in this state, lithium is considered a poor reducing agent because it requires more energy to remove an electron compared to other alkali metals.

5. **Behavior in Aqueous Medium**:
When lithium is in an aqueous medium, its behavior changes. Although it has a high ionization energy, lithium is small in size, which leads to high hydration energy when it interacts with water.

6. **Hydration Energy**:
The hydration energy is the energy released when ions are surrounded by water molecules. For lithium, this hydration energy is very high due to its small ionic size, which allows it to be extensively hydrated.

7. **Compensation of Energies**:
The high hydration energy compensates for the high ionization energy. Thus, in aqueous solution, lithium can lose an electron more readily than in the gaseous state, making it a strong reducing agent in this medium.

8. **Conclusion**:
Therefore, while lithium is a poor reducing agent in the gaseous state due to its high ionization potential, it becomes a strong reducing agent in aqueous solutions because of its high hydration energy.