Dark reactions of carbon assimilation occur in:

Explanation

The correct answer is chloroplast. Chloroplasts are the sites of photosynthesis in plant cells, encompassing both the light-dependent and light-independent reactions. The light-independent reactions, also known as the Calvin cycle or dark reactions, occur in the stroma of chloroplasts. During these reactions, carbon dioxide is assimilated into organic molecules like glucose. The other options are incorrect because they do not involve the photosynthetic process: the cytoplasmic matrix is involved in general cellular activities, leucoplasts are for storage, and mitochondria are involved in energy production through cellular respiration, not photosynthesis.