To understand why the potential drop across 199 cells is zero, let's break down the problem step by step.

1. Series Connection:
The 500 cells are connected in series, which means the positive terminal of one cell is connected to the negative terminal of the next cell, forming a loop. In a series connection, the total emf (E_total) is the sum of the individual emfs of each cell, and the total internal resistance (r_total) is the sum of the individual internal resistances of each cell.

2. Potential Drop:
In a series circuit, the total potential drop (V_total) is the sum of the potential drops across each component. Since the potential drop across each cell is the product of its emf (E) and the current passing through it (I), the total potential drop can be expressed as V_total = E_total * I.

3. Internal Resistance:
The internal resistance of the cells (r_total) also contributes to the potential drop. The potential drop across the internal resistance can be expressed as V_r = r_total * I.

4. External Resistance:
In this case, there is no external resistance connected in the circuit. Therefore, the potential drop across the external resistance is zero (V_external = 0).

5. Kirchhoff's Voltage Law:
According to Kirchhoff's Voltage Law (KVL), the sum of the potential drops in a closed loop is zero. This means that the potential drop across the cells (V_cells), the potential drop across the internal resistance (V_r), and the potential drop across the external resistance (V_external) must add up to zero.

6. Calculation:
In this scenario, since the potential drop across the external resistance is zero (V_external = 0) and the potential drop across the internal resistance (V_r) is non-zero, the potential drop across the cells (V_cells) must also be zero. Therefore, the correct answer is option D - Zero.

In summary, when 500 cells of the same emf and internal resistance are connected in series without any external resistance, the potential drop across the cells will be zero. This is because the potential drop across the internal resistance cancels out the potential drop across the cells, according to Kirchhoff's Voltage Law.

Current through the circuit