Final answer:
The true statement about MapReduce v1 APIs is that they define how MapReduce jobs are executed. They provide a way to process data using the MapReduce programming model within Hadoop's execution environment, and were not initially designed to work with YARN.
Step-by-step explanation:
The MapReduce v1 APIs are a set of interfaces and classes that were used in Hadoop before the introduction of YARN (Yet Another Resource Negotiator). The most accurate statement related to the MapReduce v1 APIs among the provided options is option A: MapReduce v1 APIs define how MapReduce jobs are executed. This is because the primary function of these APIs is to facilitate the development of MapReduce programs, which are then run within the Hadoop framework to process large datasets in a distributed fashion.
Options B and C are partially misleading. MapReduce v1 allows less flexibility compared with the later version that works with YARN. MapReduce v1 is tied closely with the execution environment provided by Hadoop and does not provide an essentially flexible execution environment (as implied by option B). As for option C, while the applications built with MapReduce v1 APIs are indeed implemented independently of the user-defined map and reduce functions, they are not largely independent of Hadoop's execution environment.
Option D is incorrect because MapReduce v1 was the original API set used with Hadoop, before YARN was introduced. It was not designed with YARN compatibility in mind, as YARN came later with MapReduce v2 to provide a more flexible resource management system.