Which of the following statements about the cells of living organisms is/are…

Cell theory holds that a plasma (cell) membrane is a non-negotiable structural feature of every living cell, prokaryotic or eukaryotic, because it is the membrane that marks a cell's boundary and controls the exchange of materials with its surroundings. A cell wall, a membrane-bound nucleus, and microscopic size are NOT universal in the same way; each of these appears only in some cell types, not in every cell.

Checking each statement in the stem against this principle: statement D (a cell membrane) is universally true, since every cell, whether bacterial, plant, animal, or fungal, is bounded by a plasma membrane. Statement B (a cell wall) fails because animal cells build no cell wall at all, unlike many plant, fungal, and bacterial cells. Statement C (a nuclear membrane) fails because prokaryotic cells, such as bacteria, keep their genetic material in a nucleoid with no membrane around it. Statement A (microscopic size) fails because a handful of single cells, the ostrich egg cell being the best-known example, are large enough to see without any magnification.

• “Only A” claims every living cell is invisible to the naked eye, a claim the ostrich egg cell alone disproves.

• “A and D” pairs a valid statement with the same microscopic-invisibility claim, so it fails on the same ground as “Only A.”

• “B and C” pairs the cell-wall claim with the nuclear-membrane claim, and both fail: animal cells have no wall, and prokaryotic cells have no nuclear membrane.

Since exactly statement D holds without exception across every cell type, the correct answer is the option that states “Only D.”