In concrete technology and quality control, the minimum compressive strength (15 cm cube) for M 30 grade concrete at 7 days should be approximately: choose the closest standard value in N/mm^2.

Introduction / Context:
Concrete does not achieve its characteristic (28-day) strength immediately. Site engineers therefore use early-age results, typically at 7 days, to monitor quality and predict 28-day outcomes. For an M 30 grade (characteristic strength 30 N/mm2 at 28 days), we identify a reasonable minimum 7-day compressive strength for standard 150 mm cubes under standard curing.

Given Data / Assumptions:

• Grade of concrete: M 30 (characteristic strength = 30 N/mm2 at 28 days).
• Specimen: 15 cm cube (150 mm).
• Age of test: 7 days under standard curing and temperature.
• Objective: select a practical minimum 7-day value used for routine control.

Concept / Approach:

Typical ordinary Portland cement concretes achieve about 60%–70% of the 28-day strength at 7 days. Applying this proportion to M 30 gives a 7-day expectation near 0.65 * 30 ≈ 19.5 N/mm2, commonly rounded and controlled around 20 N/mm2 for quick field checks and cube acceptance trends.

Step-by-Step Solution:

Verification / Alternative check:

Field correlations often use maturity or historical lab data to confirm that 7-day cubes at or above ~20 N/mm2 for M 30 mixes trend to satisfactory 28-day strengths, assuming proper curing and mix control.

Why Other Options Are Wrong:

• 7 or 10 N/mm2: Too low for M 30 at 7 days; indicates under-performance or poor curing.
• 30 N/mm2: That is the characteristic value at 28 days, not 7 days.
• 16 N/mm2: On the weak side for M 30 at 7 days under standard curing expectations.

Common Pitfalls:

• Comparing 7-day field-cured cubes (often weaker) with lab-cured standards.
• Ignoring cement type and admixtures; blended cements may gain strength more slowly.

Final Answer:

20 N/mm2