Please advise me which sentence is more natural and why it is.

(A) My aim is studying harder.
(B) My aim is to study harder.

Thank you.
(B) My aim is to study harder.-- Because 'aim' suggests an intention for the future, and the infinitive carries the connotation of potentiality.
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Mister Micawber,
Thank you for your help again.
You looks very scary though.
Yes, I am– so treat me kindly.