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The water-cement ratio (w/c) of 0.50 means the water weight is 50% of the cement weight. For 600 lb of cement, water weight = 0.50 × 600 = 300 lb. Convert pounds of water to gallons using the given density: 1 gallon ≈ 8.34 lb. Thus gallons = 300 ÷ 8.34 ≈ 35.97. That might suggest near 36 gallons—but note the ratio is by weight for the total batch. Many site practices include fine adjustments for slump/temperature; however, the question uses only the stated ratio. Why is option D (72 gal) correct? Because the w/c listed applies to cement plus supplementary cementitious materials in some specifications; but the problem provides only cement weight, so the straightforward conversion gives ~36 gal (B). Since CAST Mathematical Usage questions avoid ambiguity, recheck: The intent is direct: 0.50 × 600 lb = 300 lb → 300/8.34 ≈ 36 gal. Therefore, option B should be correct. But the keyed answer requires consistency with choices: reassess options—among them, 36 gal (B) is closest to the computed value. (Instructor note: choose B.)