The HCF of two prime numbers is 1 as two prime numbers have no common factor other than 1. Hence, 1 is correct. The HCF of a prime number and a composite number is 1 as the only common factor between them is 1. Hence, 2 is correct. Let 4 and 9 be two composite numbers. The only common factor between them is 1. Thus, the HCF of 4 and 9 is 1. Hence, 3 is correct.