A prothallus is
A. A structure in pteridophytes formed before the thallus develops
B. A saprophytic free living structure formed in pteridophytes
C. A gametophyte free living structure formed, in pteridophytes
D. A primitive structure formed after fertilisation in pteridophytes

Hint: Pro means primitive or early stage, prothallus is a structure formed in the early stage of the pteridophytes which represents the forming of germinating spores in it. They also represent a phenomenon called alteration of generation.

Complete answer:
Prothallus is found to be developed from the germinating spores of the plant
It has short life duration with a heart like structure
It has number of archegonium and antheridium growing under the prothallus
This prothallus is a small and multicellular structure
It has haploid spores in it both male and female due to which it is known to be monoecious when it has independent male or female spores it is known to be diecious prothallus.
The sporophyte grows by mitotic division
Prothallus is not dependent on the parent plant and is autotrophic in nature
Mostly contains chlorophyll
These all conclude to as gametophyte free-living structure formed in pteridophytes

Correct option is C.) A gametophyte free-living structure formed in pteridophytes.

Note: These all pteridophytes and other plants which have haploid spores as their gamete and some kind of gametophyte which are usually independent of the parent plant undergoes an alteration of generation in a pattern when fusion occurs the sporophyte is diploid in nature and the gametophytes are usually haploid in nature. They undergo this change once in the cycle mostly.