The correct answer is (b), a marriage without witness.
A marriage is considered irregular (fasid) if it does not meet the requirements of a valid marriage under Muslim law. These requirements include:
- The consent of both parties
- The presence of two witnesses
- The payment of a dowry
- The absence of any impediments to marriage, such as a prior marriage that has not been dissolved.
A marriage with a fifth wife is not irregular, as a Muslim man is allowed to have up to four wives. A marriage with a woman undergoing Iddat is also not irregular, as a woman is not allowed to remarry until her Iddat period has ended. However, a marriage without witness is irregular, as it does not meet the requirement of two witnesses.
It is important to note that a fasid marriage is not considered to be a valid marriage. This means that the couple is not considered to be husband and wife, and any children born to the union are not considered to be legitimate. However, a fasid marriage can be regularized if the couple meets the requirements of a valid marriage.