This doesn’t capture the fact that new discoveries altered scientists’ views about the ocean floor. The main point isn’t merely that scientists refined their views; it’s about what caused them to change their views. In addition, the views discussed in the passage aren’t merely about how the ocean floor was formed “many millions of years ago.” The scientists also have beliefs about how the ocean floor is currently affected.

Not supported, because the author states that basalt “was already known to locally distort compass readings on land.” This implies that there was no “discovery” of basalt’s magnetic properties; this was already something known. Since (B) is not supported, it can’t be the main point. (B) is also too narrow, since the main point isn’t focused on a new explanation for magnetic striping on the ocean floor. Rather, it’s about the fact that we’ve discovered magnetic striping patterns and the spreading of the ocean floor, and that these discoveries have changed views about the ocean floor.

This best captures the main point, which the author expresses at the beginning of the passage, and supports in the rest of the passage: “But this idea became insupportable as new discoveries were made.” “This idea” is a reference to the belief that the geology of the ocean floor had remained unchanged for millions of years. New discoveries, such as the odd magnetic variations in the ocean floor, as well as the global mid-ocean ridge, showed that belief was wrong.

Not supported and too narrow. It’s not supported because the author never connects magma to distortions of compass readings. In addition, (D) is too narrow, because it doesn’t capture the idea that new discoveries have shown that the ocean floor has not remained unchanged for millions of years.

This is too narrow, because it doesn’t capture the idea that the ocean floor has not remained unchanged for millions of years. The author’s point isn’t merely that we’ve discovered the mid-ocean ridge; it’s that the mid-ocean ridge, as well as a discovery related to magnetic variations, have changed scientists’ beliefs about the ocean floor.