The money was stolen by an employee.

An employee stole the money.

Explanation:

• An employee had stolen the money.: This active construction changes the tense from past ("was stolen") to past perfect ("had stolen"), which may alter the original meaning.
• An employee steal the money.: This active construction introduces a different tense and doesn't accurately convey the passive voice of the original sentence.
• An employee stole the money.: This active construction correctly maintains the original meaning and subject, forming the active voice.
• The money can be stolen by an employee.: This passive construction changes the tense from past ("was stolen") to present ("can be stolen") and introduces a modal verb ("can") which may alter the original meaning.