The following sentence has been split into four segments. Identify the segment that contains a grammatical error.
Kids learn / a lot by / their / school activities and homework.

The error lies in the segment "a lot by".
The preposition "by" is incorrect here. When talking about acquiring knowledge or skills through activities, processes, or tools, the appropriate prepositions are "through" or "from". The sentence should read: "Kids learn a lot through their school activities and homework" or "from their school activities...".

Thus, section D is grammatically incorrect.