Which number is a prime number?

141

161

171

151

141 (The original number); 1 + 4 + 1 = 6 (Add each individual digit together); 6 is divisible by 3; Then 141 is divisible by 3; 141 isn't a prime number; In the same way, 171 is divisible by 9; then, 171 isn't a prime number;

161 = 7 x 23; 161 is divisible by 7; then, 161 isn't a prime number;

Test whether 171 is divisible by 2, 3, 5, 7, 11, 13 (an algorithm that tests each incremental m against all known primes < division);

but, 171 isn't divisibile by 2,3,5,7,11,13;

Finally, 171 is a prime number;