The first law of thermodynamics in an isothermal process is represented by- (ΔQ = heat exchanged, Δ W = work done, and ΔU = Internal energy change).

CONCEPT:

• The first law of thermodynamics is a restatement of the law of conservation of energy. It states that energy cannot be created or destroyed in an isolated system; energy can only be transferred or changed from one form to another.
• When heat energy is supplied to a thermodynamic system or any machine:  Two things may occur:
  • The internal energy of the System or machine may change.
  • The system may do some external work.

According to the first law of Thermodynamics:

ΔQ = ΔW + ΔU

Where ΔQ = Heat supplied to the system, ΔW = work done by the system, ΔU = change in internal energy of the system 

• Isothermal process: When a thermodynamic system undergoes a physical change in such a way that its temperature remains constant, then the change is known as an isothermal process.

Change in temperature = ΔT = 0

The change in internal energy (ΔU) is given by:

ΔU = n Cv ΔT

Where Cv = molar specific heat capacity at the constant volume, n is the number of moles 

EXPLANATION:

In an isothermal process,

ΔT = 0

ΔU = n Cv ΔT = 0

According to the first law of Thermodynamics:

ΔQ = ΔW + ΔU

ΔQ = ΔW + 0 

Hence ΔQ = ΔW

So option 4 is correct.