Streptococcus thermophilus
Streptococcus penicillium
Penicillium roqueforti
Aspergillus niger
Step 1: Role of microorganisms in yoghurt production.
Yoghurt is produced through the fermentation of milk using specific bacteria. \textit{Streptococcus thermophilus} and \textit{Lactobacillus delbrueckii subsp. bulgaricus} are commonly used starter cultures.
Step 2: Explanation of other microorganisms.
Streptococcus penicillium (Option B):} Incorrect. This is not a known species used in yoghurt production.
Penicillium roqueforti (Option C):} Incorrect. This fungus is used in the production of blue cheese, not yoghurt.
Aspergillus niger (Option D):} Incorrect. This fungus is used for industrial production of citric acid, not yoghurt. \[ \therefore \text{The correct answer is: Streptococcus thermophilus