Question

.........water is essential to life (an ,the , a,)​

Answer 1

To stay alive, the organism takes in important materials for making energy, while shuttling out toxic substances such as waste products. In this regard, water is essential simply because it's a liquid at Earth-like temperatures.

Answer 2

Leave empty

Step-by-step explanation:

Water is uncountable, so definitely not "a" or "an".

We're left with either leaving it empty or "the". But "the" suggests that it's not just any water, but some type/body of water in particular, which is not the case.

Water is essential to life.