সহায়িকা

Explanation
The sentence uses the structure "not too... to", which means "not so... that it cannot be" (not extremely difficult). This construction indicates that the grammar is reasonably easy to understand. The infinitive "to understand" is the key indicator that requires the use of "too difficult".
Why other options are incorrect
ক. so difficult: The structure with so requires a that-clause (e.g., "English grammar is not so difficult that you cannot understand it").
খ. very difficult: While grammatically possible in a different sentence, "very difficult" does not fit naturally with the following "to understand" in this particular structure.
ঘ. difficult enough: This phrase would typically be used in a different context or sentence structure (e.g., "English grammar is easy enough to understand" or "English grammar is not difficult enough to cause problems"). It is not the most idiomatic fit for this specific phrase.

---