According to one acid-base theory, a water molecule acts as a base when it accepts
(1) an h+ ion (3) a neutron
(2) an oh- ion (4) an electron

Answer : The correct option is, (1) an ion

Explanation :

According to the Bronsted-Lowry or acid-base concept, a substance is called as an acid if it can donate a proton and a substance is called as a base if it can accept a proton. And the substance may be a molecule or an ion.

When a water molecule acts as an acid then it can donate a proton or ion.

When a water molecule acts as a base then it can accepts a proton or ion.

Hence, the correct option is, (1) an ion

Protons are positively charged, electrons are negatively charged, and Neutrons have no charge or are in fact "neutral". Protons and Neutrons are located in the nucleus of an atom and are most of the atoms mass. Electrons have an extremely minute mass even compared to Protons and Neutrons