<?php $category = get_the_category(); echo '<a href="'.get_category_link($category[0]->cat_ID).'" class="btn btn-default btn-lg btn-block">« Go back to ' . $category[0]->cat_name . '</a>'; ?> code example
Example: wordpress get post category name
<?php
$categories = get_the_category();
if ( ! empty( $categories ) ) {
echo esc_html( $categories[0]->name ); // Show first item of category
}
?>