Your first sentence is correct. But the details of what you are saying are confusing. Almost nowhere in the United States do cars park diagonally; they are almost always lined up vertically along the length of a street, or they are parked parallel to one another. It seems like you want to say " He stopped his car in the road diagonally to prevent other cars from passing. (like in an action movie, where the character do wild things with cars.) "Slanted" is an adjective to describe a shelf that is not perfectly level, or anything that is slightly askew. When you say the car is a matter of necessity, you sound British because of the formality of your words. An american would say... In today's world, having a car is a necessity. "The car needs to be protected" is a very general statement. Who needs to protect it? Who does the car belong to? There is so much more you could say then just "the car needs to be protected"; this statement is a gigantic understatement, and your are simply stating the obvious; an automobile is a thing of great value and of great help to us as humans, OF COURSE they need to be protected, they are worth more than $10,000! I hope this helps you. I encourage you to look up in a dictionary, every word here that you do not already know. A dictionary of your native language with english is most useful. 8]

In my opinion, neither is correct. It does depend on context, but I would use, "He parked his car at an angle to prevent other cars from passing." Why? It's just how you would say it in British English. 'Diagonally' sounds too extreme and 'slanted' would never be used in this context.

"He parked his car diagonally to prevent other cars from passing." is correct.