what kind of pipe is it? Black iron, copper tubing etc.? If black iron pipe measure across the inside diameter, if copper measure across the outside diameter.

Low point drain.

It's a talking tube that was installed by the GC for workmen to use during construction. Instead of yelling from the roof,workmen can just speak in normal tones and those inside house hear them clearly. Congress allocated 14 billion dollars to and passed the construction site noise abadement bill in 2004 requiring construction sites to cut noise level 17.8% by year 2022. Only a few smart general contractors have requested their allotment from the 14 billion so far and that's one reason the tubes arn't showing up on more new homes. Another reason the tubes aren't more common is the law alows tubes to be removed or capped once the home is complete and inspectors sign off on it. Please don't ask how i know it isn't in case an induced draft gas furnace or additional bath is added in the future. At any rate removing the cap to satisfy the inspector and buyer seems easy enough,why sweat it?

Is the pipe accesable from a basement or crawlspace? If it is cut it off there and solder a cap on. I am sorry if this isn't what you want to hear,but this is the only solution that will keep this pipe from leaking in the future. Soldering on a pipe cap is not difficult. Do a web search on soldering copper pipe and you will find plenty of material that will tell you how to do it properly. If you don't want to do it yourself,ask a friend or relative that has soldered pipe to do it for you. Or hire a plumber.................... Any other solution will eventualy leak. When it does you probably wont be home.