Hypothyroidism is a condition characterized by an underactive thyroid gland, which leads to a decrease in the production of thyroid hormones. This condition primarily affects adults and can have various effects on the body. Out of the given options, the correct answer is option 'B' - Myxoedema. Let's understand why:

Myxoedema:
- Myxoedema refers to the advanced stage of hypothyroidism in adults. It is characterized by swelling and thickening of the skin, particularly in the face, hands, and legs.
- This condition occurs due to the accumulation of fluid and mucopolysaccharides in the tissues. It can lead to a puffy appearance, dry skin, hair loss, and brittle nails.
- Myxoedema can also affect various organs, resulting in symptoms such as fatigue, weakness, depression, constipation, weight gain, and cold intolerance.

Other options and their incorrectness:

a) Addisons disease:
- Addisons disease is a condition caused by the deficiency of adrenal hormones, particularly cortisol and aldosterone. It is not directly related to hypothyroidism.
- While both conditions can affect the overall functioning of the body, they have different underlying causes and require separate treatments.

c) Sterility:
- Hypothyroidism can affect reproductive health, but it does not directly cause sterility in adults.
- In women, hypothyroidism can disrupt menstrual cycles, leading to irregular periods and potentially affecting fertility. However, it is not an absolute cause of sterility.
- In men, hypothyroidism can also affect sexual function, but again, it does not directly cause sterility.

d) Cretinism:
- Cretinism refers to a condition of severe hypothyroidism that occurs in infants and children. It is characterized by stunted growth, intellectual disability, and physical abnormalities.
- While hypothyroidism in adults can have various effects, it does not result in the same set of symptoms as cretinism.

In conclusion, hypothyroidism in adults primarily causes myxoedema, which is characterized by swelling and thickening of the skin, along with other symptoms such as fatigue, weakness, and weight gain. It is important to diagnose and manage hypothyroidism to prevent complications and improve overall well-being.