Q. Which one of the following is best Lewis base? 

Dear Student,

Lewis bases are the one which donate electrons. Among the given compounds 3 and 4 are acids as they have alcohol. 
The first one is ketone, which is having a C⁺- O^- bond. So they does not donate electron. 
The second one is ethers which is having oxygen with two lone pair of electrons and acts as a Lewis bases.
$C{H}_{3}C{H}_{2}C{H}_2\underset{..}{\overset{..}{O}}C{H}_{2}C{H}_3$

​Hence, option 2 is correct option.


Regards,