The correct matching is:

Option C

(a) $ \to $ (iii), (b) $ \to $ (iv), (c) $ \to $ (i), (d) $ \to $ (ii)

Here's a detailed explanation of the match:

(a) Thylakoids $ \to $ (iii) Flat membranous sacs in stroma: Thylakoids are indeed the flattened, sac-like structures found within the stroma of chloroplasts. They are the sites of light-dependent reactions in photosynthesis.

(b) Cristae $ \to $ (iv) Infoldings in mitochondria: Cristae are the inward folds of the inner membrane of mitochondria. These folds increase the surface area, providing more space for the electron transport chain and ATP synthesis, crucial processes in cellular respiration.

(c) Cisternae $ \to $ (i) Disc-shaped sacs in Golgi apparatus: Cisternae are the flattened, stacked membrane-bound compartments that make up the Golgi apparatus. They play a role in modifying, sorting, and packaging proteins and lipids.

(d) Chromatin $ \to $ (ii) Condensed structure of DNA: Chromatin is the complex of DNA and proteins found in the nucleus of eukaryotic cells. It condenses during cell division to form visible chromosomes.