The correct option is C (C2H5)2NH
+I effect of C2H5 group increases electron density on N and make it stronger base. So the order of basic strength would be 3o>2o>1o>NH3 but in aqueous medium presence of hydrogen bonding and bulkier groups affect the basicity. In the (C2H5)3N, alkyl groups hinder the attack of proton on N. Therefore it become less basic. And due the additive effects of steric hindrance and induction (C2H5)2NH is strongest base in aquous medium.