The correct answer is A) By draining the low point of a horizontal run of steam piping, a steam trap is used primarily to keep the steam line dry and eliminate water hammer. The primary purpose of a steam trap is to remove condensate (water) that accumulates in the steam piping system. If this condensate is not removed, it can lead to water hammer, which can damage the piping system. By placing the steam trap at the low point of a horizontal steam line, the trap can effectively drain the condensate and keep the line dry, preventing water hammer and ensuring efficient heat transfer. The other options are incorrect because they do not accurately describe the primary purpose of a steam trap. Draining the high point of a horizontal or vertical steam line would not effectively remove condensate or prevent water hammer.