Two circular coils have diameters 10 cm and 20 cm and with same number of turns. The ratio of the magnetic field induction produced at the cetnre of the coils when connected in series is

2

We know that, $B = \frac{\mu_0 ni}{2r}$

In series connection, the current is the same. Hence $B \propto \frac{1}{r}$

$\frac{B_1}{B_2} = \frac{r_2}{r_1} =\frac{2}{1}$