$RX + Mg\xrightarrow{{ether}}RMgX\xrightarrow{{C{H_3}OH}}n - Bu\tan e$
What can be R in the above reaction sequence?
A.N-butyl
B.Sec-butyl
C.N-propyl
D.Isopropyl

Hint: As we know that Grignard reagent is an organic magnesium compound which can be represented as $R - MgX$, where R refers to an alkyl or aryl group and X is the halogen. This reaction is used so as to make the hydrocarbon having a halide before, without it.

Complete answer:
Halides are very reactive substances and due to their electronegativity creates a difference in charge on it and carbon irrespective of being in a covalent bond. From the very starting those reactions or processes were very important which gave carbon and bond out of any previous compound of carbon.
How to prepare a Grignard reagent: -
These reagents are formed by the reaction of alkyl or aryl halides with magnesium in the Presence of ether. The ligands provided by ether help in stabilising the organ magnesium compound. Water and air are really harmful for the reagent.
Initially the process goes very slowly but as soon as it forms the intermediate compound, this process can be extremely exothermic. The ally and aryl fluorides aren’t used due to the high electronegativity of fluorine. The fluorine has a tendency to attract the electron cloud towards itself and thus not allowing magnesium to even react with the compound in the presence of ether so as to form an organ magnesium compound.
Now it is given that the end product is n-butane. This is a chain of four carbons and that too a straight chain. Therefore, we can say that halogen can be anywhere, among the first degree and second-degree carbons.
First degree carbons are those having one carbon attached to it while second degree carbon are those having two carbons attached to it.
Now R reacts with methanol to form n-butane, so methanol gives one of the carbons to R to form n-butane, and also the methane of methanol attaches at the first-degree position of R, so the previous halide should also be first degree and should have three carbons.
Therefore, the compound R is n-propyl.

Note:
Grignard reagents are known for their ability to quickly target carbonyls at their carbon level. Grignard reagents do not, however, function in the presence of protic solvents. Instead of reacting with the desired molecule, the Grignard is so unstable that it can readily accept a proton from a protic solvent.