The rational number in standard form is 

The standard form of rational number.

Option (A): $\frac{2}{4}$ here 2 and 4 have common factor 2

$\therefore \frac{2}{4}$ is not standard form.

Option (B): $\frac{27}{18}$ here 27 and 18 have common factor 3

$\therefore \frac{27}{18}$ is not in standard form.

Option (C): $\frac{34}{36}$, 34 and 36 have common factor 2

$\therefore \frac{34}{36}$ not in standard form

Option (D): $\frac{43}{49}$ here 43, 49, G.C.D clearly 1

There is no other factor except 1

$\therefore$$\frac{43}{49}$ in standard form