Question

Crossing over occurs when
A.) sister chromatids are pulled apart at the centromere during anaphase II.
B.) homologous chromosomes join together to form tetrads during prophase I.
C.) chromosomes are grouped together into new nuclei during telophase II.                 
D.) tetrads of homologous chromosomes randomly line up in metaphase I.

Answer 1

During meiosis 1, prophase 1 Each chromosome  form tetrads and  and become more and more condensed. This is when crossing over occurs. So your answer is B. 

Answer 2

The correct answer to this question is B:  homologous chromosomes join together to form tetrads during prophase I.