Since selection acts on phenotype, the important thing here is the extreme phenotype, which is described as advantageous, of heterozygotes. Selection is promoting one extreme, making this directional selection.
Work Step by Step
Review the definitions of the three kinds of selection mentioned. Directional selection pushes a population towards one extreme of the distribution (in terms of phenotype since these are diploid eukaryotes for which genotype and phenotype do not necessarily match up). This is what is described in the question.