The pre-emergence herbicide recommended for onion is:

Correct option is B

Oxyfluorfen is a pre-emergence herbicide used in onion cultivation to control broadleaf weeds and grasses.
(a) Pendimethalin: Also used, but oxyfluorfen is more common for onions.
(c) Paraquat: Non-selective, used post-emergence.
(d) Glyphosate: Systemic herbicide, non-selective.
Important Point: Pre-emergence herbicides are applied before weeds emerge to prevent competition.