Step 1: Understand the task.
We select the positive factors of teaching effectiveness from $J, K, L, M$.
Step 2: Check Subject knowledge (J).
Strong command of the subject ensures accuracy and clarity, so J counts.
Step 3: Check Socio-economic background (K).
A teacher's economic background does not affect teaching quality, so K is irrelevant.
Step 4: Check Communication skill (L).
Good communication ensures effective transfer of knowledge, so L counts.
Step 5: Check Classroom management (M).
Competence in managing transactions keeps the class organised, so M counts.
Step 6: Combine the valid factors.
\[ J + L + M = \text{effective set},\quad K = \text{rejected} \]
Step 7: Choose the option.
J, L and M is option D.
\[ \boxed{\text{D. J, L and M}} \]