The order of reactivities of methyl halides in the formation of Grignard reagent is

Which of the following undergoes nucleophilic substitution reaction according to the $S_{N}1$ mechanism?

The reaction $RCl + NaI \xrightarrow{\text{Acetone}} R-I + NaCl$ is known as:

In $S_{N}2$ reaction,the carbon in the transition state is

An 'Assertion' $(A)$ and a 'Reason' $(R)$ are given below. Choose the correct answer from the following options.
Assertion $(A)$: Vinyl halides do not undergo nucleophilic substitution easily.
Reason $(R)$: Even though the intermediate carbocation is stabilized by loosely held $p-$ electrons,the cleavage is difficult because of strong bonding.

The major product obtained in the following reaction is $:$