Want to populate a group of cells when a particular selection is made from a dropdown list

The formula you need is VLOOKUP:
=vlookup(Lookup_value,table_array, Column_index_num,Range_lookup)

For Weight your formula is:
=vlookup($D$5,$L$4:$P$6,2,FALSE)

For Circ your formula is:
=vlookup($D$5,$L$4:$P$6,3,FALSE)

For % vent your formula is:
=vlookup($D$5,$L$4:$P$6,4,FALSE)

For PD your formula is:
=vlookup($D$5,$L$4:$P$6,5,FALSE)