Does the average user understand the standard HTML multiple select box?

In short: No, average users don't understand that. You could either just show all options with checkboxes or try something fancy like a dropdown with checkboxes (example, see features dropdown).

1. How do you make multiple select boxes in HTML?
2. Which form element is used for multiple selection in HTML?
3. When we should allow user to select many options out of given option we should use?
4. Which tag is used to select multiple options?

How do you make multiple select boxes in HTML?

For windows: Hold down the control (ctrl) button to select multiple options. For Mac: Hold down the command button to select multiple options.

Which form element is used for multiple selection in HTML?

The <textarea> Element.

When we should allow user to select many options out of given option we should use?

Use a list box when you want to: Enable users to make multiple choices from a set of predefined options.

Which tag is used to select multiple options?

In such cases, all you have to do is use the <select> tag with the multiple attribute, and then the user will have to hold down the ctrl key on Windows, or its alternative for other OS, while making their selections.