Option 1 -> Starts with (D) which is a dependent clause beginning with "where" - cannot start a sentence.
Option 2 -> Starts with (A) which is a sentence fragment lacking a subject and main verb - grammatically incorrect.
Option 3 -> (B)-(A) creates an awkward break: "solar energy department, energy from the sun" - lacks logical connection.
Option 4 -> (B) introduces the topic, (D) explains "where" (the solar energy department), (A) continues describing what chloroplasts do with light energy, (C) completes the purpose - natural flow.
Hence, Option 4: (B), (D), (A), (C) -> Creates the complete sentence: "Easily Nature's biggest achievement comes from her solar energy department, where she has developed plant cells called chloroplast which take light energy from the sun and break down carbon dioxide and water vapor to make food, while giving out oxygen as a waste product" - this logically describes photosynthesis with proper grammatical structure and coherent flow of ideas -> correct