The correct answer is option 'C' - cementite. Cementite is the hardest and most brittle constituent of steels, making it the least strong.

Explanation:

1. Understanding the constituents of steels:
- Steels are alloys of iron and carbon, with small amounts of other elements.
- The microstructure of steels consists of different constituents that form during the cooling and solidification process.
- The main constituents of steels are ferrite, pearlite, cementite, and austenite.

2. Austenite:
- Austenite is a high-temperature phase of steel that forms at elevated temperatures.
- It has a face-centered cubic (FCC) crystal structure and is relatively soft.
- Austenite is not stable at room temperature and transforms into other constituents upon cooling.

3. Pearlite:
- Pearlite is a two-phase microstructure that consists of alternating layers of ferrite and cementite.
- It forms when austenite transforms into a mixture of ferrite and cementite during the cooling process.
- Pearlite is harder and stronger compared to austenite but is still relatively soft.

4. Ferrite:
- Ferrite is a low-temperature phase of steel that forms at room temperature.
- It has a body-centered cubic (BCC) crystal structure and is relatively soft.
- Ferrite is the softest constituent of steels and has lower strength compared to pearlite and cementite.

5. Cementite:
- Cementite is a compound of iron and carbon with a fixed chemical formula of Fe3C.
- It is a very hard and brittle phase, with a high carbon content.
- Cementite is the hardest and most brittle constituent of steels, making it the least strong.

In summary, cementite is the softest and least strong constituent of steels. It is a brittle phase with high carbon content. Ferrite is softer than cementite but still harder than austenite, while pearlite is harder and stronger than both ferrite and austenite.