the compound which does not exist is:
(A) $ B{F_3} $
(B) $ Al{F_3} $
(C) $ Al{I_3} $
(D) $ Tl{I_3} $
(E) $ Tl{F_3} $

Hint: In the question we will try to find out whether the oxidation states of the given compounds allow them to have the stability or not. Also whether the oxidation state supports their geometry or not.

Complete step by step solution:
When we talk about the boron trifluoride we come to know that it is an inorganic compound which has a pungent and a toxic smell which further burns with a white fumes in moist air. Also it further acts as a versatile building block for other boron compounds.
Then we will discuss whether the aluminium fluoride is a colorless solid and also this further acts as a building block in the production of the aluminium metal.
Then , we will talk about the aluminium iodide which is further obtained from the reaction between metallic aluminium and the hydrogen iodide as a hexahydrate. Moreover it behaves as a lewis acid and absorbs water from the atmosphere.
Talking about the thallium related compounds we come to know that the stability of the higher oxidation states decreases and the stability of the lower oxidation states increases so thallium is the most stable in executing $ + 1 $ oxidation state then talking about the thallium iodide we come to know that the $ {I_3} $ exists as a linear molecule therefore the thallium iodide is not considered to exist.
Thallium fluoride is again an inorganic compound which is white in color. It is further considered to exist in the environment.
Correct option is D.

Note:
we should note here that the stability of thallium in executing the $ + 1 $ oxidation state is due to the inert pair effect . thallium triiodide can be prepared by evaporation of the stoichiometric quantities in the concentrated aqueous solution of $ HI $ .