The tendency to lose their valance electron easily by alkali metals makes them

The correct option is "strong reducing agent".

To understand why alkali metals are strong reducing agents, let's break down the definitions and the chemical behavior of these elements:

1. Definition of Reducing and Oxidizing Agents:
An agent that donates or loses electrons during a chemical reaction undergoes oxidation itself and reduces the other reactant. Therefore, any substance that loses electrons easily acts as a reducing agent (or reductant). Conversely, a substance that gains electrons easily acts as an oxidizing agent.
This relationship can be represented as:
$$M\to M^{+}+e^{-}$$ (Oxidation of the metal, meaning it acts as a reducing agent)

2. Valence Electron Configuration of Alkali Metals:
Alkali metals belong to Group 1 of the periodic table (e.g., Lithium, Sodium, Potassium, etc.). They have a general valence shell electronic configuration of:
$$n{s}^1$$
This means they possess only one electron in their outermost shell. By losing this single valence electron, they can achieve a highly stable, noble gas-like octet configuration.

3. Ease of Electron Loss (Ionization Enthalpy):
Because they only need to lose one electron to become stable, and because they have relatively large atomic sizes with low nuclear charge shielding their outermost electron, alkali metals have very low ionization enthalpies. Consequently, they lose their valence electron extremely easily.

4. Conclusion:
Since alkali metals lose their valence electrons with great ease, they readily undergo oxidation and force other substances to gain those electrons (reducing them). This characteristic makes them exceptionally strong reducing agents.