Albert, Absolutely you can underline the important points or topics in the Physics or any other board exam, although there is no compulsion to do so, but if you can just spare a few seconds in underlining them then it just increases your presentation to another level and obviously increase your chances of scoring full marks in that particular question. See, no matter how accurate you write an answer, if the person who is going to check your copies is not able to comprehend it properly, then you can't probably score full marks in that question. However, if you can increase your presentation a touch, the copy checker can get the gist of your answer in a lesser time and it creates a nice impression in the mind of the checker. Also, there are chances that some of the smaller mistakes you make in the answer, if any, be neglected by the copy evaluator because, to be honest, the evaluators are looking for an accurate and at the same time, precise and to the point answer, so this presentation techniques may benefit you.

Hi Albert,

Yes you can do underline important points in exam. If the examiner has all the main content underlined, it would be easier for him/her to check, making him/her happy and skipping strict marking on silly mistakes.He/She might even provide you with grace marks.
I did it and I got rewarded. Stay strong, study hard and all the best for your upcoming exams.

Hope this will help,please feel free to ask if you have any further queries.

Thank You.

Yes absolutely you can underline important points in your answer so that the person who checks your answer scripts will notice your important points at first and it will be easier for him her to to evaluate your answer scripts and if he finds comfortable then definitely it will be an advantage for you.