The oxidation number of an element in a compound is a measure of the charge that element would have if all the bonding electrons were assigned to the more electronegative atom. Oxidation numbers are used to track the flow of electrons in chemical reactions and to determine the potential for redox reactions.

In pyrophosphoric acid (H4P2O7), there are two phosphorus atoms. To determine the oxidation number of phosphorus in this compound, we can use the following steps:

1. Assign oxidation numbers to the other elements in the compound:
- Hydrogen (H) is usually assigned an oxidation number of +1.
- Oxygen (O) is usually assigned an oxidation number of -2.

2. Determine the overall charge of the compound:
- The overall charge of pyrophosphoric acid is 0, since it is a neutral compound.

3. Apply the oxidation number rules to determine the oxidation number of phosphorus:
- The sum of all oxidation numbers in a compound must equal the overall charge of the compound.
- Since there are two phosphorus atoms in pyrophosphoric acid, let's assign the oxidation number of one of the phosphorus atoms as "x".

4. Calculate the oxidation number of phosphorus:
- The sum of the oxidation numbers of all the elements in the compound must equal zero. Therefore, we can write the equation:
(4)(+1) + 2(x) + (7)(-2) = 0
4 + 2x - 14 = 0
2x - 10 = 0
2x = 10
x = 5

Therefore, the oxidation number of phosphorus in pyrophosphoric acid is +5 or 5. Thus, the correct answer is option 'D'.