Asalt is best described as a compound that is formed from the reaction between

an acid and a base.
a strong acid and a wea acid.
a strong base and a weak base.
an acid and water.

An acid and a base.

Example:

HCl (acid) + NaOH (base) ---> NaCl (salt) + H2O

Salts can also be formed from the reaction between:

Acid and reactive metals:

H2SO4 (acid) + Zn (reactive metal) ---> ZnSO4 (salt) + H2

Acid and metal carbonates (also a type of base):

H2SO4 (acid) + CaCO3 (metal carbonate) ---> CaSO4 (salt) + CO2 + H2O

there is none because earth is stationary and flat.

the answer is option a.