Assertion: When $ {H_2}S $ is passed into $ FeC{l_3} $ solution yellow colour of $ FeC{l_3} $ changes to light green.
Reason: Solution changes to light green due to reduction of $ F{e^{3 + }} $ to $ F{e^{2 + }} $
(A) Both assertion and reason are correct and the reason is the correct explanation for the assertion.
(B) Both assertion and reason are correct but the reason is not the correct explanation for the assertion.
(C) The assertion is correct but the reason is incorrect.
(D) Both assertion and reason are incorrect.

Hint :the colour of a compound is dependent on the electron of the compound as the electrons of the compound gain energy and achieve an exciting level they absorb energy and coming back to ground level imparts that energy which shows colour spectrum.

Complete Step By Step Answer:
The impairment of colour is shown mostly by transition metal as they have an unfilled d-orbital they can show colours fully filled d-orbitals does not impart colour. Now iron is also a transition element since it also imparts colour but in the above question iron can be seen imparting two different colours, this is because when iron reacted with chlorine it imparted yellow colour from the solution but when iron chloride reacted with hydrogen sulphide it reduced the iron chloride from donating three electrons to donating two electrons since the number of electrons donated by the compound changed, therefore, the colour of the compound also got changed hence When $ {H_2}S $ is passed into $ FeC{l_3} $ solution yellow colour of $ FeC{l_3} $ changes to light green due to reduction of $ F{e^{3 + }} $ to $ F{e^{2 + }} $ . Therefore both assertion and reason are correct and the reason is the correct explanation for the assertion.
Option (A) is correct.

Note :
Remember only transition elements which have unfilled d-orbital can show a colour spectrum fully filled d-orbital cannot show colour spectrum. Also remember that when a compound accepts an electron or its oxidation count decreases it gets reduced, when a compound donates electron or its oxidation count increases it gets oxidised.