Hey there!
Option D would be correct.
Hope this helps, tell me if I am wrong!
I believe It’s A I’m sorry if I was wrong
On May 14, 1796, Jenner<span> took fluid from a cowpox blister and scratched it into the skin of James Phipps, an eight-year-old boy. A single blister rose up on the spot, but James soon recovered. On July 1, </span>Jenner<span> inoculated the boy again, this time with smallpox matter, and no disease developed. The </span>vaccine<span> was a success.</span>
A) Crossing over is the mechanism of genetic variation that results from the exchange of gene segments between non-sister chomatids.