Given: A cow costs more than 4 goats but less than 5 goats. The cost of a goat is between Rs. 600 and Rs. 800.

Calculations: Minimum cost of a cow: If a goat costs Rs. 600: 4 goats cost $4 \times 600 =$ Rs. 2,400. 5 goats cost $5 \times 600 =$ Rs. 3,000. Therefore, the cow's cost is more than Rs. 2,400 but less than Rs. 3,000.

Maximum cost of a cow: If a goat costs Rs. 800: 4 goats cost $4 \times 800 =$ Rs. 3,200. 5 goats cost $5 \times 800 =$ Rs. 4,000. Therefore, the cow's cost is more than Rs. 3,200 but less than Rs. 4,000.

Conclusion: Combining both scenarios, the cow's cost is between Rs. 2,400 and Rs. 4,000. Most valid conclusion: D. A cow costs between Rs. 2,400 and Rs. 4,000.