Pteridophytes and gymnosperms

Mustard is an angiosperm; its embryo sac has an egg apparatus made of one egg cell and two synergids, so the pair is correct.
$Ginkgo$ is a gymnosperm; the female gametophyte of gymnosperms bears archegonia, so the pair is correct.
Viroids are infectious agents made of free, low molecular weight RNA without a protein coat, so the pair is correct.
$Salvinia$ is a heterosporous pteridophyte: its microspores and megaspores give rise to highly reduced male and female gametophytes that develop within the spores, so it does not form a free-living, independent prothallus as homosporous ferns do.
Hence the wrongly matched pair is $Salvinia$ – prothallus.

Figure A shows a creeping, branched plant with small scale-like leaves and roots arising from the stem: $Selaginella$ (a pteridophyte, the 'spike moss').
Figure B shows an erect, jointed stem with whorls of slender branches at the nodes and a terminal cone (strobilus): $Equisetum$ (horsetail).
Figure C shows a floating aquatic fern with rounded leaves above and root-like submerged leaves below: $Salvinia$.
Figure D shows a shoot with fan-shaped leaves: $Ginkgo$, a gymnosperm.
Hence A – $Selaginella$, B – $Equisetum$, C – $Salvinia$, D – $Ginkgo$.