Endosperm develops after formation of embryo. It functions as nutritive tissue for growing embryo. The endosperm is found as liquid (coconut water) and solid (coconut kernel, white fluffy part of maize). The endosperm found in maize is of nuclear type. 

The term ?Xenia? was originally coined to refer direct effect of pollens on seed and fruit characters or effect of gene from male parent on development of fruit and seed. However, as of now, it is restricted to denote effect of pollen grains on endosperms. For example- colour of endosperm in maize is determined by origin of pollens. Option A is correct. Effect of pollen genes on fruit shape and fruit characters is known as metaxenia which makes option C incorrect. Stem is derived from plumule of embryo; likewise, vascular tissues are derived from primary cambium or procambium. Stem and vascular tissues are sporophytic structures, not products of sexual reproduction and hence, are not affected by pollen grain that is first cell of male gametophyte. Thus, correct answer is option A.