Answer
$D = 3d$
Work Step by Step
To have the same resistance, the cross-sectional area of the wire with diameter $D$ must have the same cross-sectional area as nine wires of diameter $d$.
We can find an expression for $D$:
$\pi~(\frac{D}{2})^2 = 9\pi~(\frac{d}{2})^2$
$D^2 = 9d^2$
$D = 3d$
