To calculate the weight of oxalic acid required to prepare a 1000 ml (N/20) solution, we need to understand the concept of Normality (N) and its relation to Molarity (M).

1. Normality (N):
Normality is a unit of concentration that takes into account the number of equivalents of solute present in a liter of solution. It is defined as the number of gram-equivalents of solute per liter of solution.

2. Molarity (M):
Molarity is another unit of concentration that represents the number of moles of solute per liter of solution.

The relation between Normality (N) and Molarity (M) depends on the number of equivalents of solute in a reaction. For oxalic acid (H2C2O4), the number of equivalents is 2 because it has two acidic hydrogen atoms.

Now, let's calculate the weight of oxalic acid required to prepare a 1000 ml (N/20) solution.

Given:
Volume of solution (V) = 1000 ml = 1 liter
Normality (N) = 20

The formula to calculate Normality is:
Normality (N) = (Weight of solute in grams / Equivalent weight of solute) / Volume of solution in liters

To find the weight of solute (oxalic acid), we need to rearrange the formula:
Weight of solute in grams = (Normality × Equivalent weight of solute × Volume of solution in liters)

1) Calculate the Equivalent weight of oxalic acid:
Oxalic acid (H2C2O4) has a molecular weight of 90 g/mol.
Since it has two acidic hydrogen atoms, the equivalent weight would be half of its molar weight.
Equivalent weight of oxalic acid = (Molar weight of oxalic acid / Number of acidic hydrogen atoms) = (90 g/mol / 2) = 45 g/mol

2) Calculate the weight of oxalic acid required:
Weight of oxalic acid in grams = (Normality × Equivalent weight of oxalic acid × Volume of solution in liters)
Weight of oxalic acid in grams = (20 × 45 g/mol × 1 liter)
Weight of oxalic acid in grams = 900 g

Therefore, the weight of oxalic acid required to prepare a 1000 ml (N/20) solution is 63/20 gm, which is option (C) in the given options.