The capacity in litres of a rectangular water tank with internal dimensions 8 m by 6 m by 2.5 m is:

Introduction / Context:
Capacity problems connect geometry with real life storage questions. Here, we deal with a rectangular water tank, which is just a cuboid. Once the internal dimensions are known, we can compute its volume in cubic metres and then convert that volume to litres, because capacity in practical contexts such as water storage is usually expressed in litres.

Given Data / Assumptions:

• Length l = 8 m.
• Breadth b = 6 m.
• Height h = 2.5 m.
• The tank is assumed to be fully filled with water.
• 1 cubic metre corresponds to 1000 litres.

Concept / Approach:
The internal capacity of a rectangular tank is equal to its internal volume. For a cuboid, the volume is given by:
Volume = l * b * h Once we compute the volume in cubic metres, we multiply by 1000 to convert to litres. This is because 1 m^3 of water occupies 1000 litres by definition of the litre unit.

Step-by-Step Solution:
Step 1: Compute the volume in cubic metres. Volume = 8 * 6 * 2.5. Step 2: First multiply 8 * 6 = 48. Step 3: Now multiply 48 * 2.5 = 120. So, volume = 120 m^3. Step 4: Convert cubic metres to litres. Capacity in litres = 120 * 1000 = 120000 litres.

Verification / Alternative check:
We can check the reasonableness. A 1 m by 1 m by 1 m cube holds 1000 litres. Our tank is 8 m by 6 m by 2.5 m, which has 8 * 6 * 2.5 = 120 such cubic metre units. Each contributes 1000 litres, so 120000 litres is consistent with this interpretation. The magnitude also makes sense for a fairly large storage tank.

Why Other Options Are Wrong:
120 litres corresponds to only 0.12 m^3, which is far too small for a tank of this size.
1200 litres equals 1.2 m^3 and is still extremely small compared to the actual geometric volume.
12000 litres equals 12 m^3, again much smaller than the computed 120 m^3.
60000 litres equals 60 m^3, which is exactly half the correct volume and would result from mistakenly halving one dimension or misreading 2.5 m as 1.25 m.

Common Pitfalls:
The major pitfall is mishandling the unit conversion between cubic metres and litres. Some learners mistakenly think 1 m^3 equals 100 litres instead of 1000 litres. Another error comes from incorrect multiplication of the three dimensions, especially when one dimension is a decimal like 2.5. It is always safer to multiply step by step and, if necessary, first convert 2.5 to a fraction such as 5 / 2 to simplify mental arithmetic.

Final Answer:
The capacity of the tank is 120000 litres.