Answer: I would like to apply for the position of marketing assistant and have enclosed an up-to-date copy of my CV. I have a degree in marketing and am eager to gain more experience within a demanding role.
Explanation:
The phrase "loads of experience" is too informal, and providing "just know i'm the right for the job" as the only evidence is not compelling enough.
Claiming that one feels that "they are no longer the right company for me" is not an appropriate phrase, as it shows doubt and criticism towards the hiring company.
The last option salutation is too casual. Only the selected option doesn´t have inappropriate phrases or vocabulary, and even includes information about a related degree and expectations.