Explanation:

In a diverse classroom, where learners come from different linguistic backgrounds, it is common for them to face difficulties in speaking and writing good English. The correct answer to why learners often lapse into their mother-tongue is option 'B' - they lack enough competence and the structures of the two languages are different. Let's explore this answer in detail:

Lack of Competence:
- Learners who are not proficient in English may struggle to express themselves effectively in the language.
- They may feel more comfortable using their mother-tongue as it is the language they are most fluent in.
- This lack of competence in English makes it challenging for them to communicate in the language and leads them to resort to their mother-tongue.

Differences in Language Structures:
- English and different mother-tongues have different grammatical structures, vocabulary, and pronunciation patterns.
- Learners often find it difficult to navigate these differences and may unintentionally mix elements of their mother-tongue into their English speech or writing.
- This could be due to the influence of their first language on their thinking patterns or the lack of exposure to English language conventions.

Other Factors:
- Motivation: Although option 'A' states that learners are not motivated to learn, it is not necessarily the primary reason for lapsing into their mother-tongue. While motivation can play a role, it is more likely that the lack of competence and differences in language structures are the main contributing factors.
- Ability: Option 'C' suggests that learners do not have the ability to learn English, which is an incorrect assumption. Learners in a diverse classroom may have varying levels of language proficiency, but with proper support and instruction, they can acquire English language skills.
- Slow Learners: Option 'D' assumes that learners who lapse into their mother-tongue are slow learners. However, this is not necessarily the case. The challenges faced by learners in a diverse classroom are often related to language differences rather than intellectual abilities.

In conclusion, the correct answer is option 'B' - learners in a diverse classroom find it difficult to speak and write good English because they lack enough competence and the structures of the two languages are different. By addressing these challenges through appropriate language support and instruction, learners can improve their English language skills and reduce dependence on their mother-tongue.