Which of the following numbers are prime: (a) 23 (b) 51 (c) 37 (d) 26

Answer: (a) 23 and (c) 37

Explanation: 

• 23 has only two factors 1 and 23. 
• 51 has 1, 3, 17, and 51 as its factors.
• 37 has only two factors 1 and 23. 
• 26 has 1, 2, 13, and 26 as its factors.