Almost everyone would welcome an ______ of their healthy lifespan, and some scientists are looking at increasingly extreme ways to achieve that. 1. extension 2. expansion 3. extent 4. expanse 5. expedite 6. expedient

We need a noun here as denoted by the use of the article AN (articles are used to modify only nouns). The sentence seeks to convey that almost everyone would like to live longer and some scientists are trying to make it possible but they are employing extreme measures to achieve that. EXTENSION and EXPANSION fit contextually. They both can be used to refer to the action or process of enlarging or extending something. EXTENT- the size or scale of something. EXPANSE- a wide continuous area of something EXPEDITE- make (an action or process) happen sooner or be accomplished more quickly. EXPEDIENT- (of an action) convenient and practical, although possibly improper or immoral.