The correct answer is A. The configuration parameter spark.sql.files.maxPartitionBytes directly determines the maximum size (in bytes) of a single partition when reading data from files. This setting controls how Spark splits input files into partitions during ingestion. Options B, C, and D are unrelated to initial partition sizing: B governs broadcast joins, C provides advisory sizes for adaptive query execution (AQE) during shuffles, and D sets the minimum partitions during AQE coalescing. Only A directly impacts partition size at ingestion.