The correct answer is option 'C' - the main disadvantage of intrauterine contraceptive devices (IUCDs) is that the devices can be expelled out without the knowledge of the wearers.

Below is a detailed explanation of this answer:

1. Introduction to Intrauterine Contraceptive Devices (IUCDs):
- Intrauterine contraceptive devices, commonly known as IUCDs or intrauterine devices (IUDs), are small devices that are inserted into the uterus to provide contraception.
- They are highly effective and reversible forms of birth control and are widely used by women around the world.

2. Advantages of IUCDs:
- IUCDs are highly effective in preventing pregnancy, with a failure rate of less than 1%.
- They provide long-term contraception, with some devices lasting up to 10 years.
- They do not require daily or frequent administration, unlike other forms of contraception such as oral pills.
- IUCDs do not interfere with sexual intercourse or spontaneity.
- They are suitable for most women, including those who cannot or prefer not to use hormonal methods of contraception.

3. Disadvantage of IUCDs - Expulsion:
- One of the main disadvantages of IUCDs is that they can be expelled out of the uterus without the knowledge of the wearer.
- Expulsion occurs when the IUCD is pushed out of the uterus and partially or completely expelled through the vagina.
- This can happen due to various reasons, such as uterine contractions, improper insertion, or the device not being properly anchored in the uterus.
- When an IUCD is expelled, it loses its contraceptive effectiveness, and the woman becomes at risk of unintended pregnancy.

4. Importance of Regular Check-ups:
- To minimize the risk of expulsion, it is important for women using IUCDs to have regular check-ups with their healthcare provider.
- During these check-ups, the position and placement of the IUCD can be assessed, and any necessary adjustments or replacements can be made.
- It is also important for women to be aware of the signs of expulsion, such as changes in bleeding patterns or feeling the strings of the device coming out of the cervix.
- If a woman suspects that her IUCD has been expelled, she should consult her healthcare provider for further evaluation and possible reinsertion.

In conclusion, while IUCDs are highly effective and convenient forms of contraception, their main disadvantage is the potential for expulsion without the knowledge of the wearer. Regular check-ups and awareness of expulsion signs are important to ensure the continued effectiveness of the device in preventing unintended pregnancy.