This question checks whether we can separate the jobs of primary tillage from the jobs of secondary tillage, since the four options each sound like a plausible land preparation goal.
- To prepare root bed: A loose root zone is a general result of good tillage overall, spread across both primary and secondary operations, not something that defines secondary tillage on its own.
- To break hard pan: Breaking a compacted hard pan deep in the profile calls for heavy implements like a subsoiler or chisel plough, and that work belongs to primary tillage, done before the secondary pass.
- To prepare a fine tilth seed bed: After primary tillage has already turned and loosened the soil, secondary tillage implements such as harrows, cultivators, and planks crush clods, smooth the surface, and produce fine, crumbly soil, exactly the seed bed condition needed for good seed to soil contact and even germination.
- To preserve soil structure: Keeping structure intact is more a concern tied to minimum or conservation tillage choices in general, not the stated goal of running a secondary tillage operation.
Weighing these against each other, only "to prepare a fine tilth seed bed" describes the specific job secondary tillage is meant to do.
Let's summarize:
- Primary tillage breaks rough, compact, or hard-pan soil.
- Secondary tillage refines that already-broken soil into a fine seed bed.
- General goals like root bed formation or structure preservation are broader outcomes, not the defining purpose of the secondary pass.
So option 3 is the correct answer.