Question

How are ferns more complex than mosses but still less complex than flowering plants?

Answer 1

Ferns come under the category of cryptogams in which those plants are included which are considered as the primitive forms of plants while flowering plants are more advanced plants with more complexity in structure and life processes.

Complete answer
Difference between ferns and flowering plants.

Ferns	Mosses	Flowering plants
These plants do not bear seeds and hence are called seedless plants.	These plants do not bear seeds and hence are called seedless plants.	They possess seeds and are thus called seed-bearing plants.
These are flowerless green plants that include ferns and their relatives.	They do not produce flowers and instead produce spores after fertilization.	These produce well-developed flowers that contain the reproductive parts of these plants.
These are primitive forms of plants with fewer morphological features.	They are even more primitive than ferns.	These are more advanced forms of the plant that possess complexity in both structure and function.
Reproductive structures in these plants are not well exposed and differentiated.	Reproductive structures in these plants are not well exposed and differentiated.	These have well-differentiated reproductive structures that are found on the flowers.
The embryo in these plants is naked.	The embryo in these plants is naked.	These plants have seeds within which the embryo is located along with the stored food material.
These are the first plants that have vascular tissue.	The plants lack vascular tissues.	These have well-developed and differentiated vascular tissues.
Reproduction in these plants takes place with the help of spores.	Reproduction in these plants takes place with the help of spores.	They reproduce by seeds in place of spores.
They are called cryptogams.	They come under the category of Thallophyta.	They are called phanerogams.

Note:
Cryptogams and phanerogams are two subkingdoms of Kingdom Plantae where the cryptogams include primitive plants while phanerogams are advanced plants. The cryptogams include Pteridophytes while the phaneogram includes both Angiosperms and Gymnosperms. The moose comes under the Bryophytes that do not come under Thallophyta.