Which of the following are Mendelian disorders?

(A) Cystic fibrosis
(B) Phenylketonuria
(C) Adenosine deaminase deficiency
(D) Haemophilia

Choose the correct answer from the options given below:

(A), (B) and (D) only

The correct answer is Option (2) → (A), (B) and (D) only   

Cystic fibrosis – Mendelian disorder (autosomal recessive)
Phenylketonuria – Mendelian disorder (autosomal recessive)
Haemophilia – Mendelian disorder (sex-linked recessive)

Adenosine deaminase (ADA) deficiency is generally discussed under Severe Combined Immunodeficiency (SCID)

Therefore, Option 2 is correct.