This question checks whether you can tell apart the common cropping systems used in agronomy, and asks which one is built around alternating erosion permitting and erosion resisting crops in strips.
- Mixed cropping: seeds of two or more crops are sown together with no fixed row layout, mainly as insurance against the failure of one crop, not laid out as conservation strips.
- Intercropping: two or more crops share the field in fixed row proportions to raise the total output per unit area, without any requirement to alternate erosion permitting and erosion resisting bands.
- Strip cropping: the field is split into long strips running across the slope, and strips of erosion permitting crops (like maize) alternate with strips of erosion resisting crops (like grasses or legumes), so each resistant strip checks the runoff before it can build up speed.
- Relay cropping: a second crop is planted into the first crop before it is harvested, so the two overlap in time. This is about timing, not about erosion control.
Only strip cropping matches the exact description in the question, so option (3) is correct.
Let's summarize:
- Strip cropping shortens the effective slope length by alternating resistant and permitting crop bands.
- Mixed cropping, intercropping and relay cropping are defined by crop combination or timing, not by erosion control layout.
The correct answer is strip cropping, option (3).