The age of fossils is determined through relative dating, and objective dating.

Fossils themselves usually can’t be directly dated, because most of them have little or no of the organic material left, and dating by Carbon 14 is only useful up to around 35000–50000 years. Fossils older than that must be dated by the rock layer in which they are found.

Fortunately, scientists have been studying rock strata for over 300 years, and have worked out very precisely the order in which the rock layers have been laid down. Very roughly, rocks are ranked in eras from oldest to youngest:

PreCambrian, Cambrian, Ordovician, Silurian, Devonian, Carboniferous, Permian, Triassic, Jurassic, Cretaceous, Paleogene, Neogene. Throughout the world, rocks are always found in this order, unless there’s been some obvious disturbance. This allows scientists to place a fossil found within a particular rock layer within these particular age brackets.

The only problem with this, is that knowing which layer is older than the next doesn’t give us an objective date for the rock layer. We can only know, from the above, that a cretaceous fossil is younger than a Jurassic one.

2)Brassica oleracea is the scientific name of 'wild cabbage'. By artificial selection many other species like Brussels sprout, Kohlrabi, Kale, Broccoli, Cabbage, Cauliflower have been produced form this.

As per your question

1) In case of cabbage the selected feature is 'terminal buds'.

2) In case of cauliflower the selected feature is 'flower clusters'.