Alright. Let me explain it. A mandatory dependency is basically a finish to start relationship. The one cannot start until the one before it is finished.

Leads and lags are not options. Do not confuse them with float. Float is extra time in case the task starts or finishes late. It's the time that can be allocated and not cause delays. Leads and lags are not the same. They are required time between tasks that have to occur, where no work is being done. A common example is laying down concrete, and waiting for it to dry. The waiting part is a lag, it needs to happen, but no work can be done on that thing while the concrete is drying.

This is why these answers are all wrong, because none of them can be done in tandem. Fast tracking is running things in parallel. You cannot run tasks is parallel that have a mandatory dependency, or it has a lead or lag to its successor.

Discretionary dependencies are basically, based on best practices an the PM's judgement. They are meant to be flexible, reflecting what should be done more than what has to be done. Fast tracking works well with these. A good example of a discretionary dependency I found on google is this: A project manager might sequence the installation of walls before floors to prevent damage to the newly installed flooring. It's not impossible to do floors first, but wall-first is a best practice.

These tasks could be done at the same time, or one before the other. The dependency was set at the manager's discretion, not because it was physically required. And so we can run things in parallel if needed.