Which substance yields hydroxide ion as the only negative ion in aqueous solution?

Missing question:
a. Mg(OH)2.
b. CH3Cl.
c. MgCl2 .
d. C2H4(OH)2.
Answer is: a. Mg(OH)₂.
An Arrhenius base is a substance that dissociates in water to form hydroxide ions (OH⁻).
In this example magnesium hydroxide is an Arrhenius base:
Mg(OH)₂(aq) → Ba²⁺(aq) + 2OH⁻(aq).
CH₃Cl and C₂H₄(OH)₂ are organic compound and will not give hydroxide ions in water, MgCl₂ is neutral salt and it will give Cl⁻ ions.

Yes, endothermic means it turns cold. and some chemical changes include change of tempature.