Answers for "modify the column class for the first column in the first row of the grid so that it spans across 2 out of the 12 columns of the row for small and up viewports. The two columns in the row should stack vertically for XS viewports."

col offset in bootstrap

.col-md-3 .offset-md-3

bootstrap grid

<!----------------------- BOOTSTRAP GRID SYSTEM ------------------------>

This code will create 4 boxes placed side by side, which will be 
dynamically positioned, according to the size of the screen. We define 
the following behavior: 


<!-- Desktop Display: we have 4 columns per row (each occupies 3 units out of 12)  -->

class = "col-lg-3"   <!-- display large with 12/3 = 4 columns -->


<!-- Tablet Display: we have 3 columns per row (each occupies 4 units out of 12)  -->

class = "col-md-4"   <!-- display medium with 12/4 = 3 columns -->


<!-- Mobile Display: we have 2 columns per row (each occupies the 6 units of 12)  -->

class = "col-sm-6"  <!-- display small with 12/6 = 2 columns -->


<!-- CODE -->

  <div class="row">
    <div class="col-lg-3 col-md-4 col-sm-6" style="background-color:red; border:1px solid black;">
      One of four columns
    </div>
    <div class="col-lg-3 col-md-4 col-sm-6" style="background-color:yellow; border:1px solid black;">
      One of four columns
    </div>
    <div class="col-lg-3 col-md-4 col-sm-6" style="background-color:green; border:1px solid black;">
      One of four columns
  	</div>
    <div class="col-lg-3 col-md-4 col-sm-6" style="border:1px solid black;">
      One of four columns
  	</div>
  </div>