Mark the pair of substances among the following which is essential for coagulation of blood.

Options

A. Heparin and calcuim ions
B. Calcium ions and platle factors
C. oxalates and citrates
D. Platelet factors and heparin

Explanation

To determine the pair of substances essential for the coagulation of blood, we will analyze the options provided and identify the correct ones based on the role they play in the coagulation process.

### Step-by-Step Solution:

1. **Understanding Blood Coagulation**: Blood coagulation is a complex process that involves various substances, including proteins and ions, to form a blood clot at the site of injury.

2. **Identifying Key Substances**: The main substances involved in blood coagulation include:
- Calcium ions (Ca²⁺)
- Platelet factors (such as platelet factor 3 or thromboplastin)

3. **Analyzing the Options**:
- **Heparin and Calcium Ion**: Heparin is an anticoagulant, which means it prevents coagulation rather than promoting it. Therefore, this pair is not essential for coagulation.
- **Calcium Ion and Platelet Factor**: Calcium ions are crucial for the coagulation cascade, and platelet factors (like platelet factor 3) are released by platelets at the site of injury. This pair is essential for coagulation.
- **Oxalates and Citrates**: Both oxalates and citrates are anticoagulants that bind calcium ions, preventing coagulation. Hence, this pair is not essential for coagulation.
- **Platelet Factor and Heparin**: As mentioned earlier, heparin is an anticoagulant, so this pair cannot be essential for coagulation.

4. **Conclusion**: The only pair of substances that is essential for the coagulation of blood is **Calcium Ion and Platelet Factor**.

### Final Answer:
The pair of substances essential for coagulation of blood is **Calcium Ion and Platelet Factor**.