What is a Scrum sprint?

Question $3$ options:

A planning unit in which work to be done is assessed,features are selected for planning, the software is implemented and proposed to system stakeholders.

A long $($months to years$)$ planning unit in which work to be done is assessed,features are selected for development, the software is implemented and delivered to system stakeholders.

A short $(3-4$ weeks$)$ planning unit in which work to be done is assessed,features are selected for development, the software is implemented and delivered to system stakeholders.

A short $(3-4$ weeks$)$ planning unit in which features are selected for development, the software is implemented and delivered to system stakeholders.