The correct option is
A do it
The expressions 'do it', 'do so' and 'do' are often used to avoid repeating a verb and its object.
Option B: 'Do it' refers to the action 'carry all those boxes'. 'Do' refers to the action 'carry' and 'it' refers to the object 'all those boxes'. It relates in meaning and is hence the correct option.
Option A: 'Do' expresses the action but there is no object that the action is directed towards.
Option C: 'Do so' is used to refer to general actions, and when there is no object. 'Do it' is used when the action involves a verb and its object.
There is an object here, 'boxes', hence 'do so' is not suitable.
Option D: 'These' in 'do these' is a demonstrative, it points out at a noun that is nearby. But, as mentioned in the first sentence, the object is far 'those'. Hence 'these' is not suitable.
So, options A, C and D are wrong.