Question

Photoautotrophs lacking chlorophyll $a$ are

Answer 1

Purple and green sulphur bacteria

Purple and green sulphur bacteria are photoautotrophs. During photosynthesis, hydrogen gets released from an oxidisable compound which reduces carbon dioxide and results in carbohydrate formation.

Hydrogen sulphide is the hydrogen donor instead of water. Hence, the oxidation product is sulphur and not oxygen.

Final answer:

$\left(D\right)$ Bacteria