In 1986, which spacecraft conducted a close fly-by of Halley's Comet?

The correct option is that the spacecraft that conducted a close fly-by of Halley's Comet in 1986 was notably the European Space Agency's spacecraft, specifically the Giotto probe. This mission was significant because it provided the first close-up images and detailed data about the comet, marking a major milestone in space exploration and enhancing our understanding of comets.

The other options, however, do not align with the historic events pertaining to Halley's Comet. The Soviet reconnaissance probes were primarily focused on different missions during that time and did not target Halley's Comet. NASA's Voyager probes, while critical in the exploration of the outer solar system, were not involved in the 1986 encounter with Halley’s. Mars rovers, meanwhile, are designed for exploration of Mars and have not been involved in missions regarding comets. Hence, Giotto's successful fly-by remains a landmark event in the study of comets.