The correct option is C NAD to FMN
ATP synthesis in electron transport chain in mitochondria happens due to the proton gradient and potential difference between the mitochondrial matrix and inner mitochondrial membrane.
Proton gradient is created due to the pumping of protons when high energy electrons from the electron carrier are passed in the electron transport chain.
The energy level is lowered as the electrons are transported from higher to lower energy level. This energy difference is transformed into phoshate bond energy resulting in the formation of ATP from ADP.
The electron transport begins from NADH giving its electron and proton to FMN, which then passes to FeS.
FeS gives it to UQ and UQ gives it away to cytochrome b. Cytochrome b gives it to FeS and FeS gives it to cytochrome c and it, further, gives it to cytochrome aa3. Cytochrome aa3 gives it to H2O.
When the electrons and hydrogen passes from NAD to FMN, the first ATP is formed.