The world’s largest telescope is in which country?

Concept: Telescopes are compared differently depending on type. An optical telescope's size is measured by its mirror (aperture) diameter, while a radio telescope's size is measured by its dish diameter / collecting area. When a GK question asks for the “world's largest telescope” without further qualification, it conventionally points to the largest completed single-dish radio telescope, since that instrument holds the overall record for physical collecting surface among all telescopes built.

Application: The Five-hundred-meter Aperture Spherical Telescope (FAST) is built into a natural karst depression in Guizhou province, China. Its dish is 500 metres across, making it the world's largest filled-aperture radio telescope. Its structure was completed in September 2016, after which it entered a multi-year commissioning and trial-observation phase, culminating in formal national acceptance in January 2020 — but its physical dish had already been the largest of its kind from 2016 onward, well before this question was set.

Cross-check: Checking the other offered countries on the same “which telescope's physical structure is actually the largest” test:

• India — the Giant Metrewave Radio Telescope (GMRT) near Pune is a network of thirty individual dish antennas working as an array; each dish is modest in size, valued for combined interferometric imaging rather than one single giant collecting surface.

• Japan — the Subaru Telescope (8.2 m single mirror) is a respected optical instrument, but its aperture is a small fraction of the largest collecting surfaces built elsewhere.

• United States — the Keck Observatory pairs two 10-metre segmented-mirror telescopes, historically among the largest optical instruments, while the proposed Thirty Meter Telescope remained stuck in planning and unbuilt.

• Chile — the Extremely Large Telescope (ESO), designed with a 39-metre segmented mirror, would eventually surpass every existing optical telescope once complete, but construction was still in its early structural phase, well short of first light.

Since the FAST dish in China is the only completed, largest-of-its-kind instrument among the options offered, China is the correct answer.