bootstrap4.css
.align-self-baseline {
  -ms-flex-item-align: baseline !important;
  align-self: baseline !important;
}
■レイアウト：Flex d-flex：ユーティリティ：flexを交差軸上で個別に整列 align-self-baseline
flex文字01
flex文字02（align-self-baseline）
flex文字03 
  <div class="d-flex" style="height: 100px">
    <div>flex文字01</div>
    <div class="align-self-baseline">flex文字02（align-self-baseline）</div>
    <div>flex文字03 </div>
  </div>