c
Magnetic field at $\mathrm{O}$ due to bigger coil $\mathrm{Y},$ is
$\mathrm{B}_{\mathrm{Y}}=\frac{\mu_{0}}{4 \pi} \cdot \frac{2 \pi \mathrm{i}(2 \mathrm{r})^{2}}{\left\{\mathrm{d}^{2}+(2 \mathrm{r})^{2}\right\}^{3 / 2}}=\frac{\mu_{0}}{4 \pi} \cdot \frac{8 \pi \mathrm{ir}^{2}}{\left(\mathrm{d}^{2}+4 \mathrm{r}^{2}\right)^{3 / 2}}$
Magnetic field at $\mathrm{O}$ due to smaller coil $\mathrm{X}$ is
$\mathrm{B}_{\mathrm{x}}=\frac{\mu_{0}}{4 \pi} \cdot \frac{2 \pi \mathrm{ir}^{2}}{\left\{\left(\frac{\mathrm{d}}{2}\right)^{2}+\mathrm{r}^{2}\right\}^{3 / 2}}=\frac{\mu_{0}}{4 \pi} \cdot \frac{16 \pi \mathrm{ir}^{2}}{\left(\mathrm{d}^{2}+4 \mathrm{r}^{2}\right)^{3 / 2}}$
$\Rightarrow \quad \frac{\mathrm{B}_{\mathrm{Y}}}{\mathrm{B}_{\mathrm{X}}}=\frac{1}{2}$