Here's why:

* Chloroplasts are the organelles within plant cells and some protists (like euglena) that contain chlorophyll.

* Chlorophyll is the pigment that allows for photosynthesis, the process by which organisms convert light energy into chemical energy (sugars).

* Autotrophs are organisms that can produce their own food through photosynthesis.

Therefore, the presence of chloroplasts in an euglena indicates its ability to perform photosynthesis and thus its classification as an autotroph.