Hey there!
Your answer is C, checking grammar and spelling.
Let's see why this is right and the other options aren't.
For A, using slang isn't professional. It makes you come off as careless and not very interested in writing a serious application you can be proud of. For B, typing responses, it's something that people indeed do, but it doesn't necessarily make or break your application. If your handwriting is good, filling in the application by hand is just as good. For D, we need to keep quality over quantity in mind. Creating lengthy responses might add a lot of detail, but it might not have a lot of quality, and all those pages may just be unnecessary information. That's why C is correct. If you have all your grammar and spelling correct, it makes it look like you know what you're talking about and are able to effectively complete a job application.
Hope this helps!