To solve this problem, we need to determine the largest excess of rice over wheat that can be achieved using a budget of Rs. 2800, with the condition that only whole packs can be bought. Let’s analyze the options available:

The goal is to find the combination of packs such that the difference between the amount of rice and wheat is maximized under the condition of the given budget.

Calculation:

1. First calculate maximum amount of any pack that can be bought with Rs. 2800:
   • Pack A: \(2800 \div 400 = 7\)
   • Pack B: \(2800 \div 500 = 5\)
   • Pack C: \(2800 \div 600 \approx 4\)
2. Calculate Rice - Wheat difference for 1 unit of each pack:
   • Pack A: \(3 - 4 = -1\) kg
   • Pack B: \(4 - 3 = 1\) kg
   • Pack C: \(5 - 4 = 1\) kg
3. To maximize the difference, buy combinations which have a net positive balance for Rice - Wheat:
   • Focus first on Pack B and C wherever possible as they both have a positive net difference.
   • One possible combination under the budget: 4 Pack B (Rs. 2000) + 1 Pack C (Rs. 600) = Rs. 2600 total
4. Rice - Wheat for 4 Pack B and 1 Pack C:
   • Total Rice in 4 Pack B and 1 Pack C: \((4 \times 4) + 5 = 21\) kg
   • Total Wheat in 4 Pack B and 1 Pack C: \((4 \times 3) + 4 = 16\) kg
   • Difference: \(21 - 16 = 5\) kg

The largest excess of rice over wheat that can be purchased is thus 5 kg. Therefore, the correct option is 5 kg.