What is a characteristic of an analytic sandbox?

A study was run to identify general dietary patterns among the residents of a small town. Twelve thousand people were surveyed and the data was subject to K-means clustering.
In one of the iterations, there were six clusters formed with 38, 1560, 1799, 2560, 2893, and 3150 respondents.
What should be the next step in identifying optimal clusters?

Refer to the exhibit.

What is the approximate R-squared value for a linear regression model fitted to the data associated with this scatterplot?

When creating a presentation for a technical audience, what is the main objective?

Before you build an ARMA model, how can you tell if your time series is weakly stationary?