Bootstrap/scss/_utilities.scss

@import "utilities/background";
@import "utilities/center-block";
@import "utilities/clearfix";
@import "utilities/pulls";
@import "utilities/screenreaders";
@import "utilities/spacing";
@import "utilities/text";
@import "utilities/visibility";

@if $enable-flex and $enable-grid-classes {
  @import "utilities/flex";
}