Correct option is (a) 325
To elaborate: To choose any clock for the first pick, there are 10+16=26 options. For the second choice, we have 25 clocks left to choose from and so on. Thus, by the rule of product, there are 26 * 25 * 24 * 23 = 650 possible ways to choose exactly four clocks. However, we have counted every clock combination twice. Hence, the correct number of possible ways are 650/2 = 325.