fatter sociable weaver birds spend more time foraging where they are vulnerable to predation, are more attractive to predators, and are less likely to escape an attack. conversely, thin birds risk starvation. therefore, sociable weaver of an intermediate weight have a selective advantage. this is an example of selection. a. frequent
b. directional
c. disruptive
d. stabilizing
e. mutational

d. stabilizing

Explanation:

When the intermediate phenotypes have a better survival fitness and are better adapted to the prevailing conditions, natural selection favors them. Over the generations, the organisms having the intermediate phenotype reproduce more and leave more progeny. In this way, the proportion of the intermediate phenotype in the population is increased over time. This process is called stabilizing selection.

In the given example, the sociable weavers with intermediate body-weight have a selective advantage as they easily escape predators with their relatively light body and can withstand the starvation. Therefore, natural selection increases their proportion over generations. This represents an example of stabilizing selection.  

layer 3 is younger than layer 6.