Hello Varsha!!
In CBSE boards there are two types of diagram questions,  first one is where they already give you a diagram and you have to identify the marked areas and the second one is where you dram and label the diagrams.
Diagrams often comes from the first unit ie Reproduction one so you can first prepare that. Remember every label in diagram, because you get marks for labelling diagram not for drawing.
Diagrams in the Biotechnology unit are also important. From the other units diagrams come in boards.
Hope this helps!
All the best!