Final answer:
Sharding is a database partitioning technique that divides data horizontally, where each shard contains a subset of the rows but all columns of the tables, improving performance in distributed systems.
Step-by-step explanation:
Sharding is a data partitioning method that divides large databases to spread the load, thereby improving performance in distributed database systems. The process involves breaking down data into smaller, faster, more easily managed parts, known as shards. Each shard is an independent database, and together, the shards make up the larger database. The correct answer to the question is A. horizontally. When a dataset is sharded horizontally, each shard holds a subset of the rows from the database's tables, yet all columns of the table are preserved. This differs from vertical partitioning, where different database tables may be separated, or different columns of a table are divided.