Which of the following contextual classes are used to color the panels?

d. All of the above
 
Explanation: Bootstrap 4 provides a panel that can have a bordered box with some padding around its content. In order to color the panel, a user has to use contextual classes which are given below:

1. .panel-default – it gives the default styling to the panel in a web page
2. .panel-primary – it informs the user, that the panel is important and gives it a color of blue.
3. .panel-success – it informs the user, that the panel has some successful data and gives it a color of green
4. .panel-info – it informs the user to look at the information closely and gives it a color of lighter shade of blue.
5. .panel-warning – it informs the user that the panel contains some warning signs and gives it a color of yellow.
6. .panel-danger – it informs the user that this panel has some potentially harmful information which may be dangerous and gives it a color of red.