The correct answer is option 'D': four inorganic sulfides and washing with an acid.

Explanation:
Cubane is a hydrocarbon molecule with eight carbon atoms arranged at the corners of a cube, and it is often used as a structural motif in inorganic chemistry. Ferredoxin is a type of protein that contains cubane-like clusters of iron and sulfur atoms.

1. Inorganic sulfides in cubane-like ferredoxin:
Inorganic sulfides are sulfur atoms that are bonded to inorganic elements, such as metals. In cubane-like ferredoxin, there are four inorganic sulfides present. These sulfides are coordinated with iron atoms in the cubane cluster.

2. Removal method:
The removal of inorganic sulfides from cubane-like ferredoxin can be achieved by washing with an acid. Acidic conditions can facilitate the dissociation of the inorganic sulfide bonds with the iron atoms, leading to their removal from the protein.

3. Acidic washing:
The acidic washing involves treating the cubane-like ferredoxin with an acid solution. The acid can protonate the sulfide ions, making them more susceptible to dissociation from the iron atoms. This process can be done by adjusting the pH of the solution to an acidic range, typically below pH 7.

4. Acid-base reactions:
The acid-base reactions that occur during the acidic washing can be represented by the following equations:
- Acid (H+) + Sulfide (S2-) → Hydrogen sulfide (H2S)
- Iron-sulfide bond (Fe-S) + Acid (H+) → Iron ion (Fe2+) + Hydrogen sulfide (H2S)

5. Acid washing vs. base washing:
While both acid and base washing can be used to remove inorganic sulfides, the choice of the washing method depends on the specific system and the desired outcome. In this case, acid washing is the appropriate method because it is more effective at dissociating the inorganic sulfides from the cubane-like ferredoxin.

In conclusion, the number of inorganic sulfides in cubane-like ferredoxin is four, and the appropriate method for their removal is washing with an acid.