The correct option is
A He has a funny laugh.
Few words can be used as both nouns and verbs. When a word is used as an action word, it is called a verb. When a word is used as the main subject of the sentence, it becomes a noun.Here, we have to find out a sentence, where 'laugh' is used as a noun. Hence, 'laugh' should be the subject/object of the sentence and not an action.
Option B - 'I laughed so much at that joke' is a wrong choice. Here, the subject 'laughed', which is an action word. Hence, option B is incorrect.
Option C - 'I'm still laughing' is a wrong choice. 'Still' explains that a person is doing something for a long time. As 'laughing' is an action word, it is not a noun. Hence, option C is incorrect.
Option D - 'Laugh as much as you can because it is good for health' is a wrong choice, as the subject (not given/silent) asks us to 'laugh' do something (action). Hence, option D is incorrect.
Option A - 'He has a funny laugh' is the right choice, as the adjective 'funny' describes how the 'laughter' is. Here, 'laugh' is the object (noun). Hence, option A is the correct answer.