What happens in this reaction? butan-1-amine + ch3i -->

Here we have to get the product between the reaction of butane-1-amine with methyl iodide (CH₃I).

The  reaction between 1 mole of butan-1-amine and 1 mole of methyl iodide produces Methyl-butamine which is a secondary amine.

However, In presence of 2 moles of methyl iodide the reaction proceed to N, N-di-methylbutamine. The reaction is shown in the figure.

This is one of the effective reaction method to generate secondary and tertiary amine from primary amine.

The primary amine reacts with alkyl iodide to form secondary to tertiary amine. The final product depends upon the quantity of the alkyl iodide present in the reaction.

the answer is option a.