• # question_answerA) 21                       B)       25C) 50                       D)       60E) None of these

Explanation Option [b] is correct. Square of number at the bottom is equal to the product of two numbers at the top. i.e., ${{6}^{2}}=4~9,\text{ }i.e.\text{ }36$ ${{12}^{2}}=9~\times 16,\text{ }i.e.\text{ }144$ ${{20}^{2}}=16\times ?,\text{ }i.e.\,400$ The missing number is $400~\div 16=25$