The correct answer is option 'C', which states that the straight grades of cemented carbide cutting tool materials contain tungsten carbide and cobalt. Let's understand why this is the correct answer.

Tungsten Carbide:
- Tungsten carbide (WC) is the primary component of cemented carbide cutting tool materials.
- It is an extremely hard and wear-resistant material, making it ideal for cutting tools.
- Tungsten carbide provides the necessary hardness and strength to the cutting tool.

Cobalt:
- Cobalt (Co) is used as a binder or matrix material in cemented carbide cutting tools.
- It acts as a binding agent, holding the tungsten carbide particles together.
- Cobalt also provides toughness and improves the overall strength of the cutting tool.

Combination of Tungsten Carbide and Cobalt:
- The combination of tungsten carbide and cobalt in cemented carbide cutting tools offers a balance of hardness and toughness.
- Tungsten carbide provides the hardness required for cutting applications, while cobalt improves the toughness and strength of the tool.
- This combination allows the cutting tool to withstand high cutting forces and resist wear.

Advantages of Tungsten Carbide and Cobalt Combination:
- Excellent wear resistance: Tungsten carbide is one of the hardest materials known, and when combined with cobalt, it forms a cutting tool material that can withstand high wear conditions.
- High hardness: Tungsten carbide provides the necessary hardness for cutting various materials, including steel, cast iron, and non-ferrous alloys.
- Good toughness: Cobalt improves the toughness of the cutting tool, allowing it to withstand impacts and prevent chipping or breakage.
- High temperature resistance: Cemented carbide cutting tools can withstand high cutting temperatures without losing their hardness or wear resistance, thanks to tungsten carbide and cobalt.

In conclusion, the straight grades of cemented carbide cutting tool materials contain tungsten carbide and cobalt. This combination provides the necessary hardness, toughness, and wear resistance required for cutting applications.