Step 1: Understanding the Question.
Sentence i, "The problem of food is intimately connected with population", is the fixed opener and sentence vi, "they fall an easy prey to all sorts of diseases.", is the fixed close. We arrange ii, iii, iv and v between them.
Step 2: Key Approach.
This paragraph is a cause and effect chain in economics. Work out which sentence states the general principle, which applies it, and which sentence supplies the antecedent for "they" in the fixed last sentence.
Step 3: Starting the chain.
Sentence i links food to population but does not explain how. Sentence iii gives the general economic rule that explains the link: "the market is governed by demand and supply". A general principle comes before its application, so iii is the first middle sentence.
Sentence v applies that rule: "if too many people demand goods to go round, prices will rise and poor classes will starve". Too many people is the demand side, rising prices is the supply side, so v follows iii.
Step 4: Closing the chain.
Sentence ii, "wages will seldom rise in proportion to the rising prices", picks up "rising prices" straight from v and explains why the poor cannot cope. So ii follows v.
Sentence iv, "without enough food, such people lack health, strength of efficiency", uses "such people", pointing back to the poor classes who cannot afford food. It is the last middle sentence, and "such people" then becomes "they" in the fixed sentence vi.
Step 5: Reading the full paragraph and checking the rest.
i, iii, v, ii, iv, vi runs: food is tied to population, the market runs on demand and supply, too many people push prices up and the poor starve, wages do not keep pace, those people lack health without food, and so they fall ill. The middle order is iii, v, ii, iv.
Option (B) ii, iii, iv, v mentions "rising prices" in ii before any price rise has been introduced, and ends on v, so "poor classes will starve" is followed by "they fall an easy prey", which skips the health link but more importantly leaves iv stranded before v with no antecedent for "such people".
Option (C) iv, ii, v, iii opens with "such people" right after i, where no group of people has been described, and buries the demand and supply principle at the end where it explains nothing.
Option (D) v, iii, iv, ii applies the rule before stating it and ends on ii, so "wages will seldom rise in proportion to the rising prices, they fall an easy prey to all sorts of diseases" leaves "they" without a referent.
Final Answer:
The correct sequence of the middle sentences is iii, v, ii, iv, which is option (A).