Answer
$7 \times 10^8 m$.
Work Step by Step
The subtended angle (in radians) is the solar diameter (i.e., twice the solar radius) divided by the Earth to sun distance.
$$(0.5^{\circ})\frac{2 \pi rad}{360^{\circ}}=\frac{2R}{1.5 \times 10^{11}m}$$
Solve for $R \approx 7 \times 10^8 m$.
