In Mirabilis jalapa , flower colour exhibits incomplete dominance....

In Mirabilis jalapa, flower colour shows incomplete dominance.

Let the allele for red colour be $R$ and for white colour be $r$.

The genotypes and their corresponding phenotypes are:
$RR$ : Red
$Rr$ : Pink
$rr$ : White

To obtain a progeny with only pink ($Rr$) and white ($rr$) flowers in a $1:1$ ratio, the cross must be between a heterozygote and a homozygous recessive plant.

Crossing a pink-flowered plant ($Rr$) with a white-flowered plant ($rr$):
$Rr \times rr \rightarrow 1 Rr$ (Pink) : $1 rr$ (White)

Thus, a cross between a pink-flowered plant and a white-flowered plant yields the desired $1:1$ phenotypic ratio.

Answer: Cross between a pink-flowered plant and a white-flowered plant