Does a Carnot engine working under an isothermal state perform any useful work?

Which statement is incorrect?

In a Carnot engine,if the absolute temperature of the source is $25 \%$ more than the absolute temperature of the sink,then the efficiency of the engine is (in $\%$)

$A$ diatomic ideal gas is used in a Carnot engine as a working substance. If during the adiabatic expansion part of the cycle,the volume of the gas increases from $V$ to $32V$,the efficiency of the engine is:

$A$ car engine has a power of $20 kW$. The car makes a round trip of $1$ hour. If the thermal efficiency of the engine is $40 \%$ and the ambient temperature is $300 K$, the energy generated by fuel combustion is: (in $kJ$)

The Carnot cycle (reversible) of a gas represented by a Pressure-Volume $(P-V)$ curve is shown in the diagram. Consider the following statements:
$I.$ Area $ABCD =$ Work done by the gas
$II.$ Area $ABCD =$ Net heat absorbed
$III.$ Change in the internal energy in the cycle $= 0$
Which of these are correct?