Choose the composite number amongst the following.

Composite numbers are numbers that have more than two factors.

Here, 51 has more than two factors. 

$51=3\times 17$

$51=1\times 51$

So, Factors of 51 are 1, 3, 17, and 51. 

All other numbers have only two factors, 1 and the number itself.

So, 51 is a composite number.