Final answer:
The study's sampling method is best described as stratified sampling.
Step-by-step explanation:
The study's sampling method can best be described as stratified sampling. Stratified sampling involves dividing the population into distinct groups, or strata, based on certain characteristics, and then randomly selecting samples from each stratum.
In this case, the soccer coach is selecting players from different age groups to form a team, which represents the use of stratified sampling.