Step 1: Understanding the meaning.
The original sentence emphasizes that understanding main ideas is the most important factor — not optional or secondary. Therefore, the correct alternative must express necessity.
Step 2: Evaluating the options.
(A) Too strong and repetitive with "unless and until," though close in meaning.
(B) Correct — It clearly states that possessing the ability is required to develop strong reading skills.
(C) Uses “might,” which weakens the meaning, contradicting “most important factor.”
(D) Adds “solely,” which overstates the idea not supported in the original.
Step 3: Conclusion.
Option (B) matches the original meaning by expressing necessity without exaggeration.