Thumbnail image is not fitting the entire column when resizing - html

Question

I have included a sidebar in one column and 3 thumbnails in another 3 columns in a row. The thumbnail image is fitting the entire columns at first but when I resize the thumbnail images are getting smaller keeping the column in full width (as for usual bootstrap responsive works). I need to stretch my images as that of the column they are filled in.
Here's my HTML:
<link href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-beta/css/bootstrap.min.css" rel="stylesheet"/>
<div class="container-fluid side-navigation-bar">
<div class="row">
<!--==========================================-->
<!--Side navigation bar-->
<!--==========================================-->
<div class="col-md-3 col-md-3-thumbnail">
<div class="nav-side-menu" >
<div class="brand"><i class="fa fa-heartbeat"> myVitals.com</i></div>
<i class="fa fa-bars fa-2x toggle-btn" data-toggle="collapse" data-target="#menu-content"></i>
<div class="menu-list" >
<ul id="menu-content" class="menu-content collapse out" >
<li>
<a href="#">
<i class="fa fa-home fa-lg fa-fw"></i> Home
</a>
</li>
<li>
<a href="#">
<i class="fa fa-book fa-lg fa-fw"></i> My Appointments
</a>
</li>
<li>
<a href="#">
<i class="fa fa-s15 fa-lg fa-fw"></i> Facilities
</a>
</li>
<li>
<i class="fa fa-cogs fa-lg fa-fw"></i> Services
</li>
<li>
<i class="fa fa-stethoscope fa-lg fa-fw"></i> Doctors
</li>
<li>
<a href="#">
<i class="fa fa-medkit fa-lg fa-fw"></i> Medications
</a>
</li>
<li>
<a href="#">
<i class="fa fa-medkit fa-lg fa-fw"></i> Medications
</a>
</li>
</ul>
</div>
</div>
</div>
<!--==========================================-->
<!--End of Side navigation bar-->
<!--==========================================-->
<!--==========================================-->
<!--Cards-->
<!--==========================================-->
<div id="column" class="col-md-3 col-sm-6 col-md-3-thumbnail text-center">
<div class="thumbnail">
<figure class="imghvr-hinge-down">
<img src="http://via.placeholder.com/320x211" alt="Bootstrap Thumbnail Customization">
</figure>
<div class="caption">
<h5><b>Bootstrap Cards Design</b></h5>
<p class="card-description">Hi there How are you?</p>
<p>Read More</p>
</div>
</div>
</div>
<div id="column" class="col-md-3 col-sm-6 col-md-3-thumbnail text-center">
<div class="thumbnail">
<figure class="imghvr-hinge-down">
<img src="http://via.placeholder.com/320x211" alt="Bootstrap Thumbnail Customization">
</figure>
<div class="caption">
<h5><b>Bootstrap Cards Design</b></h5>
<p class="card-description">Hi there How are you?</p>
<p>Read More</p>
</div>
</div>
</div>
<div id="column" class="col-md-3 col-sm-6 col-md-3-thumbnail text-center">
<div class="thumbnail">
<figure class="imghvr-hinge-down">
<img src="http://via.placeholder.com/320x211" alt="Bootstrap Thumbnail Customization">
</figure>
<div class="caption">
<h5><b>Bootstrap Cards Design</b></h5>
<p class="card-description">Hi there How are you?</p>
<p>Read More</p>
</div>
</div>
</div>
</div>
</div>
I even tried the width:100% for the thumbnail image as
.thumbnail > img{width:100%; display:block;}
The problem is, the img is not taking the full-width as that of its column
Can anyone please help me???

Answer 1

Used .img-fluid for responsive image in Bootstrap.4. it have following css:
.img-fluid {
max-width: 100%;
height: auto;
}
https://getbootstrap.com/docs/4.0/content/images/

Answer 2

You can just set the width to 100% and the height to auto, this will expand it to full width.
CSS:
.thumbnail > figure > img {
width: 100%;
height: auto;
}
JSFiddle Demo