Agroup of students performing an experiment collect euglena samples from a pond and keep them in darkness in a vessel containing pond water
which would be the conclusion for this experiment? a) the euglena cells remain alive because they are completely heterotrophic
and feed on the organic matter present in the pond water. b) the euglena cells die because they are completely autotrophic
in nature and are unable to create food in the absence of sunlight.
c) the euglena cells, which are both autotrophic and heterotrophic, remain alive even in the absence of sunlight because
they can feed upon the organic material present in the pond water.

Option C, The Euglena cells, which are both autotrophic and heterotrophic, remain alive even in the absence of sunlight because  

they can feed upon the organic material present in the pond water.

Explanation:

Euglena is a special organism as it possesses the characteristics of both autotrophic and heterotrophic species. In the presence of light the chloroplast with in the Euglena entraps energy from sun and use it in the process of photosynthesis to make its own food. But in the absence of sunlight , euglena is capable to absorb food through its cell membrane and thus in this way they can also behave as heterotroph.  

Thus Euglena placed in darkness in a vessel containing pond water will survive.