5. It's a lovely day, isn't it? The sun _________ (shine). Let's go for a walk in the park.

5. It's a lovely day, isn't it? The sun is shining. Let's go for a walk in the park.

В данном предложении используется Present Continuous, так как действие (солнце светит) происходит в данный момент времени.

Ответ: is shining