**Answer:**

Construction of a massive concrete dam requires careful consideration of various factors such as strength, durability, and heat generation during curing. The most suitable type of cement for this purpose is **low heat cement** (option C).

**1. Low heat cement:**
Low heat cement is a type of cement that generates less heat during the hydration process compared to other types of cement. It is specially designed to minimize the risk of thermal cracking and reduce the temperature rise within the concrete mass. This makes it ideal for large-scale concrete structures like dams.

**Advantages of low heat cement in dam construction:**

- **Reduced heat generation:** The hydration process of cement produces heat, which can cause thermal stresses in large concrete structures. Low heat cement reduces this heat generation, minimizing the risk of thermal cracking and ensuring long-term structural integrity of the dam.

- **Control of temperature rise:** The temperature rise within a concrete dam can have detrimental effects on its performance. Low heat cement helps to maintain a lower temperature rise during curing, preventing potential damage and maintaining the desired strength and durability.

- **Better workability:** Low heat cement often exhibits better workability properties, allowing for easier placement and compaction of concrete. This is beneficial during the construction of a massive dam, as it ensures proper consolidation of the concrete and reduces the likelihood of voids or weak spots.

- **Long-term durability:** Low heat cement contributes to the long-term durability of the dam by reducing the risk of cracking and increasing resistance to chemical attacks. This ensures that the dam remains structurally sound and functional for its intended lifespan.

**Other options:**

- **Ordinary Portland cement (option A):** Ordinary Portland cement is a commonly used type of cement, but it may not be the most suitable choice for a massive concrete dam due to its higher heat generation during hydration.

- **Rapid hardening Portland cement (option B):** Rapid hardening cement is designed to gain strength quickly, but it may not be the best choice for a dam as it can generate high heat and may not provide sufficient time for proper curing and consolidation.

- **Blast furnace slag cement (option D):** Blast furnace slag cement is a blended cement that contains slag, which can improve the durability and workability of concrete. While it has its advantages, it may not be specifically designed to control heat generation, making low heat cement a more suitable option for a massive concrete dam.

In conclusion, low heat cement is the most appropriate choice for constructing a massive concrete dam due to its reduced heat generation, control of temperature rise, better workability, and long-term durability.