To identify the four curves on a $P$–$V$ diagram, we use the standard shapes of thermodynamic processes.

1. Recall the characteristic graphs

For a gas on a $P$–$V$ diagram:

1. Isochoric process: volume remains constant.

   • Hence it is a vertical line.

2. Isobaric process: pressure remains constant.

   • Hence it is a horizontal line.

3. Isothermal process: $PV=\text{constant}$.

   • This is a rectangular hyperbola.

4. Adiabatic process: $PV^\gamma=\text{constant}$.

   • This is also a falling curve, but it is steeper than the isothermal.

2. Match these with the labels in the figure

From the standard diagram:

• the vertical line is labeled $d$ $\Rightarrow$ isochoric = $d$
• the horizontal line is labeled $a$ $\Rightarrow$ isobaric = $a$
• among the two falling curves, the less steep one is isothermal and is labeled $b$ $\Rightarrow$ isothermal = $b$
• the steeper one is adiabatic and is labeled $c$ $\Rightarrow$ adiabatic = $c$

So, in the order:

3. Check the options

• A: $d\ a\ b\ c$ ✅
• B: $a\ d\ b\ c$ ❌
• C: $d\ a\ c\ b$ ❌
• D: $a\ d\ c\ b$ ❌

Thus the correct option is:

$\boxed{A}$

4. Comparison with stored answer

Stored correct answer = A

My derived answer = A

Hence, they agree.