The correct option is A Eternal
'Having no beginning or end to its existence' is called Option A, Eternal. Option B, 'Obscure' means unclear or unknown. Option C, 'Universal' means existing everywhere. Option D, 'Immeasurable' means so large that it cannot be measured or known exactly. Thus, the options B, C and D are incorrect and the correct word for the given word is Option A, Eternal.