Yes you can distribute one line of code as "open source software" by assigning that one line of code an OSI Approved Open Source License.
You should only label your software as "open source software" (i.e. tell everyone it is open source) if you distribute it with an OSI Approved License. Calling it open source is meaningless--the license determines if it is open source software, not a marketing campaign.
The license you assign to your work not only determines if it is open source software, but--per your second question--how others may distribute derivatives of your work: either only as open source software (for example through the GPL family of licenses) or with proprietary software (for example through the Apache, BSD, or MIT licenses).
So if you distribute your one line of code with a GPL-like "copyleft" license, anyone who modifies your one line will also need to distribute their edits under the same, open source (GPL) license.
If, for example, you were to assign the BSD License to your one line of code, anyone can take that line of code and include it with their proprietary software and even sub-license it (as long as they include the copyright information in their distribution).
Any software distributed with an OSI Approved Open Source License is "open source software."
Software NOT distributed with an OSI Approved License, but labelled as "open source software" should be considered suspect.
A great resource for learning about various open source licenses and their permissions/restrictions is, TL;DR Legal