This is a challenging passage. The ocean floor spreading theory argues that mid-ocean ridges mark structurally weak zones where the ocean floor is being pulled apart and new magma erupts through these weak zones, creating new oceanic crust (lines 33-40). The passage cites several pieces of evidence supporting the theory - (1) the rocks are very young at or near the ridge crest; (2) the youngest rocks all have normal polarity (since we would expect the most recent magma to have the same polarity as the current earth's magnetic field);(3) correlation between the ages of the magnetic reversals and the striping pattern.
The question asks us which of the following if true would support the ocean floor spreading theory. Notice that one of the pieces of evidence cited in support is that geophysicists had assigned ages to the earth's recent magnetic reversals by measuring the magnetic orientation of continental volcanic rocks (lines 46-49) and concluded that the ages of magnetic reversals coincided with the striping pattern. This is not a very strong piece of evidence on its own and (B) strengthens it by pointing out that the ages of the earth's magnetic reversals have been also verified by other means other than examining magnetite grains in rock. An answer choice to strengths a piece of evidence or provides additional evidence for a theory is commonly a correct answer choice for the RC support/ strengthen question.
If we look at the rest of the answer choices, none of them have any impact on the ocean floor spreading theory.
(A) is irrelevant because the theory is supported by the evidence of polarity of basalt rocks. The passage is not concerned with other types of rocks. (C) is irrelevant because the passage is only concerned with submarine basalt not the one found on land. (D) is irrelevant because the passage never said that the height is important; (E) is irrelevant and is similar to (A), the passage is not concerned with rocks nearest to the continents.
Let me know if this helps and if you have any other questions.
