Explain the following statement: Scale to a primary six student.

Step 1: Identify the map scale and the measured distance.

The scale is $1 : 50000$, meaning $1~cm~on~map = 50000~cm~actual.$

Measured distance on map between towns: $4~\mathrm{cm}$.

Step 2: Calculate actual distance in cm.

Formula: $\mathrm{Actual~distance~(cm)} = Map~distance~(cm) \times 50000$

Substitute: $4 \times 50000 = 200000~cm.$

Step 3: Convert cm to km (keep units).

$200000~cm = 200000 \div 100000 = 2~km.$

Step 4: Check student statements (assume standard options):

• Student A: $200~\mathrm{m}$ (incorrect)
• Student B: $20~\mathrm{km}$ (incorrect)
• Student C: $2000~\mathrm{m}$ (incorrect)
• Student D: $2~\mathrm{km}$ (correct)
• Student E: $20000~\mathrm{cm}$ (incorrect)
• Student F: $0.2~\mathrm{km}$ (incorrect)

Student D is correct.

$\boxed{Student D}$