which number is prime?\n49\n51\n53\n55

Answer

A prime number has only two distinct positive divisors: 1 and itself.

$49 = 7\times7$, so it has divisors 1, 7, and 49, not prime.

$51=3\times17$, so it has divisors 1, 3, 17, and 51, not prime.

Test divisibility from 2 to $\lfloor\sqrt{53}\rfloor\approx7$. It is not divisible by 2, 3, 4, 5, 6, 7. So 53 is prime.

$55 = 5\times11$, so it has divisors 1, 5, 11, and 55, not prime.