According to Faraday’s laws of electrolysis, to which of the following is the amount of decomposition proportional to?

The correct option is The electrochemical equivalent of the substance.

Faraday's First Law of Electrolysis:
According to Faraday's first law of electrolysis, the mass of a substance deposited, liberated, or decomposed at any electrode during electrolysis is directly proportional to the quantity of electricity passed through the electrolyte.

If $m$ is the mass of the substance decomposed and $Q$ is the quantity of electricity (in coulombs) passed, then:

$m\propto Q$

Since the quantity of electricity is the product of current ($I$) and time ($t$) for which the current passes, we can write:

$Q=I·t$

Substituting this in the proportionality relation:

$m\propto I·t$

To convert this proportionality into an equation, we introduce a constant of proportionality:

$m=Z·I·t$

Here, $Z$ is the constant of proportionality known as the electrochemical equivalent of the substance.

From this equation, for a given quantity of electricity passed ($I·t=\text{constant}$), the mass of the substance decomposed ($m$) is directly proportional to its electrochemical equivalent ($Z$):

$m\propto Z$

Hence, the amount of decomposition is directly proportional to the electrochemical equivalent of the substance.