a tag without line-decoration in Bootstrap 4

Bootstrap 4 has the class "text-decoration-none"

<a href="#" class="text-decoration-none">no underline</a>

https://getbootstrap.com/docs/4.3/utilities/text/#text-decoration

There are several Bootstrap 4 classes that set text-decoration: none. Without knowing the semantic context of the link, I suggest you use the .btn class...

<a href="#" class="text-dark btn">a tag with text-dark class and css to disable underline on hover</a>

https://www.codeply.com/go/CIt6zI0Iqw

However, there may be a class that works better semantically such as nav-link (inside nav), card-link (inside cards), or .breadcrumb-item. Also, note that Bootstrap 4 only applies the underline to anchor links with the href="" attribute. A simple anchor anchor will not have an underline on hover.