The product of two rational numbers is 

For example,

$\frac{2}{3},\frac{-5}{7}\in Q$

$\text{Product\hspace{0.17em}}=\frac{2}{3}\times \left(\frac{-5}{7}\right)=\frac{-10}{21}\in Q$

Every integer is a rational number.

$2,\frac{-5}{3}\in Q\Rightarrow \text{product}=2\times \left(\frac{-5}{3}\right)=\frac{-10}{3}\in Q$

$-2,4\in Q\Rightarrow \text{product}=-2\times 4=-8\in Q$

Since $-8$ integer and also a rational number.

i. $2,3\in Q\Rightarrow \text{product}=2\times 3=6\in Q$ since 6 is natural number also. E
ii. $-2,5\in Q\Rightarrow \text{product}=-2\times 5=-10\in Q$

But $-10$ is not natural number.

So, always natural number is incorrect.

Sometimes a natural number is correct

So, both a and b are correct