diff --git a/core/modules/book/tests/src/Functional/BookTest.php b/core/modules/book/tests/src/Functional/BookTest.php
index 354b887..53afc1a 100644
--- a/core/modules/book/tests/src/Functional/BookTest.php
+++ b/core/modules/book/tests/src/Functional/BookTest.php
@@ -584,7 +584,7 @@ public function testBookOutline() {
     $this->drupalLogin($this->adminUser);
     $this->drupalGet('node/' . $empty_book->id() . '/outline');
     $this->assertRaw(t('Book outline'));
-//    $this->assertOptionSelected('edit-book-bid', 0, 'Node does not belong to a book');
+    $this->assertOptionSelected('edit-book-bid', 0);
     $this->assertNoLink(t('Remove from book outline'));
 
     $edit = array();
@@ -624,7 +624,7 @@ public function testBookOutline() {
 
     // Test the form itself.
     $this->drupalGet('node/' . $node->id() . '/edit');
-//    $this->assertOptionSelected('edit-book-bid', $node->id());
+    $this->assertOptionSelected('edit-book-bid', $node->id());
   }
 
   /**
diff --git a/core/tests/Drupal/FunctionalTests/AssertLegacyTrait.php b/core/tests/Drupal/FunctionalTests/AssertLegacyTrait.php
index 20f16c5..ad42806 100644
--- a/core/tests/Drupal/FunctionalTests/AssertLegacyTrait.php
+++ b/core/tests/Drupal/FunctionalTests/AssertLegacyTrait.php
@@ -217,6 +217,26 @@ protected function assertPattern($pattern) {
   }
 
   /**
+   * Asserts that a select option in the current page is checked.
+   *
+   * @param string $select
+   *   The locator of the select field to assert.
+   * @param string $option
+   *   Option to assert.
+   *
+   * @deprecated Scheduled for removal in Drupal 9.0.0. Use methods provided by
+   *   \Drupal\Tests\WebAssert instead, for example:
+   * @code
+   *   $option = $this->assertSession()->optionExists($select, $value);
+   *   $this->assertTrue($option->getAttribute('selected'));
+   * @endcode
+   */
+  protected function assertOptionSelected($select, $option) {
+    $node = $this->assertSession()->optionExists($select, $option);
+    $this->assertTrue($node->getAttribute('selected'));
+  }
+
+  /**
    * Pass if the page title is the given string.
    *
    * @param string $expected_title