A virus can sometimes be regarded as a living organism because it

Step 1: Analyze the question The question asks why a virus can sometimes be regarded as a living organism. So we need the one feature that viruses share with living things.

Step 2: The concept Viruses are borderline between living and non-living:

• Non-living features: They have no cells, no cytoplasm, and no organelles. They do not respire, grow, or carry out metabolism on their own. Outside a host they can be crystallised and are inactive.
• Living features: Once inside a living host cell, they use the host's machinery to reproduce (multiply). They also contain genetic material (DNA or RNA) and can mutate and evolve.

Step 3: Check each option

• A. Causes diseases in plants and animals. Disease-causing is not unique to living things. Toxins and chemicals can cause disease too, so it doesn't show a virus is alive.
• B. Exists in a variety of shapes. Shape is not a characteristic of life. Non-living crystals also come in many shapes.
• C. Easily visible with an electron microscope. This is just about size and observation, not a life process.
• D. Reproduces in living cells. Reproduction is a key characteristic of living organisms, and viruses can do it, but only inside a host cell. This is why they are sometimes regarded as living.

Step 4: Answer The correct option is D. Reproduces in living cells.

Exam tip: Remember viruses as "alive only in a host." When a question asks why a virus may be considered living, look for reproduction or multiplication inside host cells. When it asks why they are non-living, look for no cellular structure or crystallisation outside a host.