Which stage of plasmodium vivax is infective to mosquito?
(A) Sporozoite (B) Gametocyte (C) Merozoite (D) Zygote
(B) When certain forms of blood stage parasites (gametocytes, which occur in male and female forms) are ingested during blood feeding by a female Anopheles mosquito, they mate in the gut of the mosquito and begin a cycle of growth and multiplication in the mosquito. 
After 10-18 days, a form of the parasite called a sporozoite migrates to the mosquito’s salivary glands. 
When the Anopheles mosquito takes a blood meal on another human, anticoagulant saliva is injected together with the sporozoites, which migrate to the liver, thereby beginning a new cycle.

Ref :https://www.cdc.gov/malaria/about/biology/index.html