Which of the following amines will be most reactive when treated with HCl?

N-Ethylethanamine

Out of the methyl substituted and ethyl substituted aqueous amines, the latter will have relatively higher K_b values due to the ethyl group being larger than methyl group. Out of the ethyl substituted amines, the secondary amine will be the most basic because of the combined inductive and solvation effect of alkyl group. Thus, being the most basic, it will react faster with HCl.