Step 1: Calculate the total man-power output first. With the standard value of 0.1 hp per laborer under ideal conditions, 20 laborers together give $20 \times 0.1 = 2$ hp of total power.
Step 2: Multiply by the working duration of 6 hours to get the energy consumed: $2 \times 6 = 12$ hp-h, which is exactly option (C).
Step 3: Check consistency by converting this same energy into kilowatt-hours using $1 \text{hp} = 0.746 \text{kW}$: $12 \times 0.746 \approx 8.94$ kWh, which is option (A). Since both values describe the identical amount of energy, only options (A) and (C) can be correct together; (B) at 120 kWh and (D) at 240 hp-h are roughly ten to twenty times too large to be right.
\[\boxed{\text{(A) and (C) only}}\]