6. The dog (not bark) today.

Задание 6. The dog (not bark) today.

Present Perfect negative. Подлежащее 'The dog' (единственное число, третье лицо) требует 'has'. Третья форма глагола 'bark' — 'barked'.

Получаем:

The dog has not barked today.

Ответ: has not barked