Basic strength of different alkyl amines depends upon:
A. $ + I$ effect
B. steric effect
C. solvation effect
D. all of these

Hint: We know that amines are compounds which contain one or more than one basic nitrogen atom with a lone pair. When amines are attached to any compound which contains alkyl group $R$ then it is known as alkyl amine. The basic strength of different alkyl amines depends on many factors and effects.

Complete step by step answer:
> The alkyl group donates electrons to the nitrogen atom because it is more electronegative. The strength of donating lone pairs of electrons increases as the number of alkyl groups increases. Hence the basic strength of alkyl amines depends on $ + I$ effect.
> When atoms in a molecule get too close together as part of the molecule vibrate or rotate, electric repulsions between electrons may hinder some conformations and favour others, this is known as steric effect. Due to steric effect alkyl groups crowd around the nitrogen atom which hinders the attack of proton on the amine. Hence the basic strength of molecules decreases.
> Solvation is an interaction of solute with the solvent which stabilizes the solute species in the solution. > This effect is known as the salvation effect. The solution of ammonium salt of primary and secondary amines undergoes a salvation effect because of hydrogen bonding.
> Hence the basic strength of different alkyl depends upon at all the given effects so option D is the correct answer to this problem that is all of these.

So, the correct answer is “Option D”.

Note:
1- We now know that alkyl amine possesses an alkyl group and basic nitrogen atom .So all the given effects can be applied in the determination of the basic strength of alkyl amines.
2- We can say that the basicity of amines do not depend on a single factor or effect.