Let's carefully analyze the definitions provided:
1. Acts as proton acceptor in any system: This is not the correct definition of an Arrhenius acid. Rather, this describes a Bronsted-Lowry base, which accepts protons.
2. Increases [tex]\(\left[ H ^{+}\right]\)[/tex] when placed in water: This is the correct definition of an Arrhenius acid. An Arrhenius acid is a substance that increases the concentration of hydrogen ions [tex]\(\left[ H^{+}\right]\)[/tex] in aqueous solution.
3. Acts as proton donor in any system: This describes a Bronsted-Lowry acid, which donates protons.
4. Decreases [tex]\(\left[ H^{+}\right]\)[/tex] when placed in water: This describes a base, as bases reduce the hydrogen ion concentration in water either by directly accepting [tex]\( H^{+}\)[/tex] ions or by generating [tex]\( OH^{-} \)[/tex] ions which neutralize [tex]\( H^{+}\)[/tex] ions.
Given these definitions, the correct choice is:
Increases [tex]\(\left[ H^{+}\right]\)[/tex] when placed in water.
Thus, the answer to the question "An Arrhenius acid is defined as a substance which:" is:
Increases [tex]\(\left[ H^{+}\right]\)[/tex] when placed in water.
