Explanation:

- Marriage with a fifth wife: Under Muslim Law, a man cannot have more than four wives at a time. Marrying a fifth wife is not just irregular, it is void (batil), so not classified as "irregular."

- Marriage with a woman undergoing Iddat: This marriage is considered irregular (fasid). Marrying a woman who is still in her waiting period is not permitted under Muslim Law.

- Marriage contracted without witness: According to Sunni Law, marriage without witnesses is irregular (fasid). Under Shia Law, witnesses are not mandatory, so this rule mainly applies to Sunnis.

- Correct Answer: Only marriage with a woman undergoing Iddat and marriage without witnesses are irregular. Marriage with a fifth wife is void, not irregular.