Blue poppies native to china were grown at a plant-breeding center in california. the plants with the thickest leaves were most likely to survive and reproduce in the drier climate. after several generations, the percentage of thick-leaved plants had increased by 42%. this adaptation of the poppies to their new environment is due to

Directional selection

Explanation:

The type of natural selection that favors one of the extreme phenotypes due to its higher survival and reproductive success is known as directional selection. It shifts the peak of the distribution curve of the phenotype towards one extreme end.  

In the given example, the plants with thickest leaves have better survival and reproductive success under the drier climate of their new habitat. This would allow the plants with thickest leaves to survive well and leave more progeny.

After several generations, the population would exhibit an increased frequency of plants with thickest leaves over the plants with intermediate and thinnest leaves.

Since the natural selection has favored one extreme phenotype (thickest leaves) over the average (intermediate thickness) and the other extreme (thinnest leaves) phenotype, it is directional selection.