Choose the equation of reaction that correctly represents anaerobic respiration in muscles:

NCERT states: 'the first step is the break-down of glucose... into a three-carbon molecule called pyruvate. This process takes place in the cytoplasm... Sometimes, when there is a lack of oxygen in our muscle cells, another pathway for the break-down of pyruvate is taken. Here the pyruvate is converted into lactic acid which is also a three-carbon molecule. This build-up of lactic acid in our muscles during sudden activity causes cramps.'

Option D — Correct. $\text{Glucose} \rightarrow \text{Pyruvate} \xrightarrow{\text{Lack of oxygen}} \text{Lactic acid} + \text{Energy}$ exactly matches this description: glucose breaks down to pyruvate in the cytoplasm, and in the absence of sufficient oxygen in muscle cells, pyruvate is converted to lactic acid, releasing a small amount of energy.

Option A — Incorrect. This equation (pyruvate to ethanol + CO2 in absence of O2) describes anaerobic respiration in yeast during fermentation, not in human muscle cells.

Option B — Incorrect. This shows glucose being converted to pyruvate and then to energy entirely under $O_2$, which describes aerobic respiration, not an anaerobic muscle pathway; it also omits any carbon by-product.

Option C — Incorrect. This mixes conditions inconsistently (pyruvate formed using $O_2$, then converted to ethanol under lack of oxygen) — ethanol is not the muscle-cell anaerobic product, and the first arrow incorrectly shows oxygen being required for glycolysis, which actually occurs in the cytoplasm independent of oxygen.