how to vertically align div in bootstrap code example

Example 1: bootstrap vertical align

<div class="d-flex align-items-start">...</div>
<div class="d-flex align-items-end">...</div>
<div class="d-flex align-items-center">...</div>
<div class="d-flex align-items-baseline">...</div>
<div class="d-flex align-items-stretch">...</div>

Example 2: bootstrap column vertical align

<span class="align-baseline">baseline</span>
<span class="align-top">top</span>
<span class="align-middle">middle</span>
<span class="align-bottom">bottom</span>
<span class="align-text-top">text-top</span>
<span class="align-text-bottom">text-bottom</span>

Example 3: bootstrap table text vertical align center

/* Based on what you have provided your CSS selector is not specific enough to 
override the CSS rules defined by Bootstrap.

Try this:
*/

.table > tbody > tr > td {
     vertical-align: middle;
}

/*
In Boostrap 4, this can be achieved with the .align-middle Vertical 
Alignment utility class.
*/
<td class="align-middle">Text</td>