Provide the reagents necessary to carry out the following conversion.
1. Na2Cr2O7/H2SO4/H2O2. Cl2, light (hν)3. (CH3CH2)2CH2MgBr4. H2O1. Na2Cr2O7/H2SO4/H2O2. Cl2/FeCl33. CH3CH2MgBr4. H2O1. Na2Cr2O7/H2SO4/H2O2. SOCl23. excess CH3CH2MgBr4. H2O1. Na2Cr2O7/H2SO4/H2O2. SOCl23. (CH3CH2)2CH2MgBr4. H2O

see explanation below

Explanation:

First, you are not providing the reaction to do the conversion. However, in picture 1, I attached a reaction that match perfectly with the description of this question, so I hope it could be.

Now picture 2, shows you the correct steps of conversion, now let me explain a little here the mechanism:

The first step in all options is the same. Those reagents works to do an oxidation of the methyl group to a carboxilic acid, and then, we transform toluene into benzoic acid. Now the second step in the first option involves Cl2 in light, and this only works to do halide alkylation in acyclic chains, so this would not work in the reaction to get the final product. In option 2, we have Cl2/FeCl3, and this will promote a friedel craft alkylation, and Chlorine will go to the para position of the ring, so we can exclude this option. However option 3 and 4 uses SOCl2, and this is often used to convert the hydroxile group of a molecule into an halide, so these two options works in the second step.

Now, for the step 3, option 4 uses grignard reagent with (CH3CH2)2CH2MgBr as reagent, but this will only form a larger chain and the CH3CH2 will not break, so, this option is discarted, and finally option 3 will be the correct one. Picture 2 shows you every step.

the answer is a and have a great day