HTML

card button align bottom bootstrap

Add d-flex flex-column to card-body class,
Add mt-auto to button class.


<div class="card-body d-flex flex-column">
 <h2>Plan Name</h2>
  <p>Feature 1</p>
  <p>Feature 2</p>
  <p>Feature 3</p>
  <button type="button" class="mt-auto btn btn-lg w-100 btn-outline-dark">Sign Up</button>