The task is to inspect a welded aluminium structure for both surface and internal defects while keeping health risk to operators minimal. Two material facts govern the choice: aluminium is non-ferromagnetic, and the safest methods avoid ionising radiation. A complete inspection needs one method for surface defects and one for internal (sub-surface) defects.

Why dye penetrant plus ultrasonic testing is correct:

• Dye Penetrant Testing (DPT) reveals surface-breaking defects such as cracks and surface porosity. A coloured or fluorescent liquid is drawn into surface openings by capillary action and then made visible with a developer. It works on any non-porous material — including aluminium — and uses only chemicals, so there is no radiation hazard.
• Ultrasonic Testing (UT) sends high-frequency sound waves into the metal and detects echoes from internal flaws (voids, inclusions, lack of fusion). It penetrates the full thickness, so it covers internal defects, and it uses harmless sound energy rather than radiation.
• Together this pair covers surface (DPT) and internal (UT) defects with minimal operator health risk.

Why the other combinations fail one of the requirements:

• Any option containing radiographic testing uses X-rays or gamma rays, which are ionising radiation — a significant health hazard — so it violates the "minimal health risk" requirement.
• Any option containing magnetic particle testing relies on magnetising the part, which only works on ferromagnetic materials; aluminium is non-magnetic, so this method simply cannot inspect it.
• A purely visual plus magnetic-particle pairing also cannot reveal buried internal defects and again fails on the non-magnetic aluminium.