Step 1: Identify the topic.
The question asks which statements about the Pluta vowel (Plut swar) are correct among $J, K, L, M$.
Step 2: Recall the nature of Pluta.
Pluta is a three-matra elongated vowel, mainly heard in Vedic chanting and in calling out from a distance.
Step 3: Test statement J.
$(J)$ It has no standard independent letter in modern Devanagari and is marked using a special notation; this is correct in modern script usage.
Step 4: Test statement L.
$(L)$ It was used extensively in the Vedic language for meter and rhythm; this is historically true, so $L$ is correct.
Step 5: Reject K and M.
$(K)$ claims it is simply not used in Hindi, but it survives in forms like Om, so it is not fully correct. $(M)$ calls it a one-matra vowel, yet it is three-matra, so $M$ is wrong.
Step 6: Combine the correct ones.
Only $J$ and $L$ stand, which is option B.
\[ \boxed{\text{B. } J \text{ and } L} \]