The question asks for the Bouguer anomaly of a landmass. We are given the thickness, density, and a specific value for the constant $ \pi G $. The Bouguer anomaly ($ \Delta g_B $) for a horizontal slab is calculated using the formula:

$ \Delta g_B = - 2 \pi G \rho h $

Where:

• $ \rho $ is the density of the landmass.
• $ h $ is the thickness of the landmass.
• $ \pi G $ is given as $ 21 \text{ mgal/km/g/cc} $.

Bouguer Anomaly Calculation Steps

1. Identify Given Values:
   • Thickness, $ h = 2.0 \text{ km} $
   • Density, $ \rho = 2.7 \text{ g/cc} $
   • Constant, $ \pi G = 21 \text{ mgal/km/g/cc} $
2. Apply the Formula: Substitute the values into the Bouguer anomaly formula. The negative sign is standard for the Bouguer plate correction representing the gravity effect of the excess mass.

   $ \Delta g_B = - 2 \times (21 \text{ mgal/km/g/cc}) \times (2.7 \text{ g/cc}) \times (2.0 \text{ km}) $

3. Calculate the Anomaly: Perform the multiplication.

   $ \Delta g_B = - 2 \times 21 \times 2.7 \times 2.0 \text{ mgal} $

   $ \Delta g_B = - 42 \times 5.4 \text{ mgal} $

   $ \Delta g_B = - 226.8 \text{ mgal} $

4. Round to Nearest Option: The calculated value $ -226.8 \text{ mgal} $ is approximately $ -227 \text{ mgal} $.

Final Bouguer Anomaly Value

The calculated Bouguer anomaly is approximately $ -227 \text{ mgal} $, which corresponds to option 4.