Which of the following is gram-positive bacillus-
Listeria is gram-positive bacillus.
Staphylococcus and streptococcus are also gram-positive, but they are cocci (not bacilli).
Meningococci are gram-negative cocci.

Also know

Clinically important spore-forming bacteria are gram-positive → Clostridium, Bacillus.
Listeria is the only gram-positive bacteria that has endotoxin. (Endotoxin is present only in gram-negative organisms).
Of the gram-negative organisms, only two groups are cocci - Neisseria (meningococcus & gonococcus), veillonella.
All other gram-negative organisms are bacilli (including coccobacilli).
Amongst medically important bacilli '6' are gram (+)ve → Actinomycetes, clostridium, mycobacteria, Bacillus, Listeria, Corynebacterium.
All other medically important bacilli are gram-negative.
Staphylococcus
Streptococcus
Listeria
Meningococci