Step 1: Recall the opposite convention used in structural or solid mechanics for steel and concrete members, where tension is usually taken as positive because engineers are often interested in tensile failure of ductile or reinforced materials.
Step 2: Soil behaves completely differently. Because soil particles simply rest against each other with friction and interlocking and are not bonded like a solid, they cannot transmit a real tensile pull between grains, so any attempt to place soil in tension gives essentially zero or meaningless stress in the field.
Step 3: Since a soil mass in the ground is squeezed by its own weight and by whatever load sits above it, essentially every normal stress a soil element experiences in practice is a compressive push, confirming Reason (R) as a true physical statement.
Step 4: Given that compressive stress is effectively the only kind of normal stress soils really carry, it is far more convenient for engineers to define that common, dominant case as positive, which is exactly the convention stated in Assertion (A). This shows the reason given directly causes and explains the convention, not merely accompanies it as an unrelated fact.
Step 5: Both A and R are true, and R correctly explains why A is the adopted convention, so the answer is option 1.\[\boxed{\text{Both (A) and (R) true; (R) is the correct explanation of (A)}}\]