The problem describes the MacArthur-Forrest cyanide process for the extraction of gold. We need to identify the reagents and products involved in the two main steps of this process.

Step 1: Leaching of Gold

The first step involves leaching the gold ore with a dilute solution of sodium cyanide (NaCN) or potassium cyanide (KCN). This is an oxidation process where gold is converted into a soluble complex. An oxidizing agent is required for this reaction, which is typically oxygen from the air.

The balanced chemical equation for this step is: $4 \text{Au(s)} + 8 \text{CN}^-(\text{aq}) + \text{O}_2(\text{g}) + 2 \text{H}_2\text{O(l)} \rightarrow 4 [\text{Au(CN)}_2]^-(\text{aq}) + 4 \text{OH}^-(\text{aq})$

According to the question, gold is leached with CN$^-$ in the presence of Q to form R.

• Comparing this with the reaction, the substance present along with CN$^-$ is oxygen. Therefore, Q is O$_2$.
• The soluble complex formed is the dicyanoaurate(I) ion. Therefore, R is [Au(CN)$_2$]$^-$.

Step 2: Precipitation of Gold

The second step is the recovery of gold from the soluble complex solution. This is done by a displacement reaction using a more electropositive metal, typically zinc (Zn). Zinc reduces the gold complex, causing metallic gold to precipitate out.

The balanced chemical equation for this step is: $2 [\text{Au(CN)}_2]^-(\text{aq}) + \text{Zn(s)} \rightarrow [\text{Zn(CN)}_4]^{2-}(\text{aq}) + 2 \text{Au(s)}$

According to the question, R is treated with T to obtain Au and Z.

• We identified R as $[\text{Au(CN)}_2]^-$. It is treated with zinc metal. Therefore, T is Zn.
• The products are metallic gold (Au) and a soluble zinc complex. Therefore, Z is [Zn(CN)$_4$]$^{2-}$.

Step 3: Evaluating the Options

Now we evaluate each option based on our findings:

• A: Q is O$_2$. This is correct. Oxygen acts as the oxidizing agent in the leaching step.
• B: Z is [Zn(CN)${}_4$]${}^{2-}$. This is correct. Zinc forms the stable tetracyanozincate(II) complex.
• C: T is Zn. This is correct. Zinc metal is the reducing agent used to displace gold.
• D: R is [Au(CN)${}_4$]$^-$. This is incorrect. The stable complex formed by Gold(I) with cyanide is the linear dicyanoaurate(I) ion, $[\text{Au(CN)}_2]^-$. The coordination number is 2, not 4.

Therefore, the correct options are A, B, and C.