Which type of transport is responsible for oxygen entering into blood cells?

a. passive
b. active
c. vesicle
d. facilitate

Passive is 100% correct ! its job is to let oxygen enter into blood cells

For each of the examples, simple diffusion, carrier, and channel mediated, each type involves the flow down a concentration gradient. This means that the molecules flow from an area of high concentration to an area of low concentration. In turn, this allows for a passive process, meaning no ATP is needed! I hope this explains your question.