Step 1: Understanding the Question.
Sentence i opens the paragraph and sentence vi closes it. We order ii, iii, iv and v in between so the paragraph tells one connected story about atomic energy.
Step 2: Key Approach.
Three clues do the work: the connector "but" in ii needs a positive statement before it, the pronoun "it" in iv needs a singular noun to refer back to, and the contrast word "however" in the fixed last sentence needs a gloomy statement just before it.
Step 3: Fixing the first two middle sentences.
Sentence i praises the release of atomic energy as science's greatest achievement. Sentence iii carries on in the same positive tone: "the atom was split by physicists whose minds were set on the search for knowledge". So iii follows i.
Sentence ii begins with "but" and reverses that tone: "the first invention to which their discoveries were applied was a bomb". The word "their" also points back to "physicists" in iii, which confirms that ii comes straight after iii.
Step 4: Fixing the last two middle sentences.
Sentence iv says "it was more deadly than any other weapon invented so far". The pronoun "it" needs an antecedent, and the only singular noun that fits is "a bomb" at the end of ii. So iv follows ii.
Sentence v then gives the scientists' reaction: "it is with dread that scientists regard the first use to which their greatest discovery was put". Dread is the natural response to a deadly weapon, so v follows iv.
Finally vi opens with "however" and swings back to the bright side, balancing the dread in v. That confirms v sits right before vi.
Step 5: Reading the full paragraph and checking the rest.
i, iii, ii, iv, v, vi reads: the achievement, how it happened, the grim turn with "but", how deadly it was, the scientists' dread, and then "however" for the peaceful uses. The middle order is iii, ii, iv, v.
Option (A) ii, iii, iv, v opens with "but" immediately after i, so the bomb appears before the physicists who are referred to as "their", leaving the pronoun without an antecedent.
Option (B) v, iii, ii, iv puts the dread before the bomb is even mentioned and ends on iv, so "it was more deadly than any other weapon invented so far, however, they are gratified..." leaves "they" hanging.
Option (D) iv, v, iii, ii starts with "it" right after i, so "it" would have to mean atomic energy rather than the bomb, and the paragraph ends on the "but" clause, which cannot be followed by "however".
Final Answer:
The correct sequence of the middle sentences is iii, ii, iv, v, which is option (C).