Which of the following code snippets will not throw an error?

Question

In the code of my Drupal 8 module *beautiful*, what are the correct ways to display a *message* to the user in good practice?

Accepted Answer

drupal_set_message(t('Candidate %firstname %lastname was added.', ['%firstname' => $firstname, '%lastname' => $lastname]));

Accepted Answer

drupal_set_message($this->t('Candidate %firstname %lastname was added.', ['%firstname' => $firstname, '%lastname' => $lastname]));

Accepted Answer

drupal_set_message(t('Candidate %firstname %lastname was added.', array('%firstname' => $firstname, '%lastname' => $lastname)));

Answer

drupal_set_message(t('Candidate ' . $firstname . ' ' . $lastname . ' was added.'));

Answer

drupal_set_message($this->t('Candidate !firstname !lastname was added.', array('!firstname' => $firstname, '!lastname' => $lastname)));

Answer

drupal_set_message($this->t('Candidate ' . $firstname . ' ' . $lastname . ' was added.'));

Question from the Drupal 8 test