Correct Answer: Only II

Solution : Statement I: The ratio of the population of S1 to the population of S2 in the year Y1 are 3 : 1.
From the graph,
Population of S1 in Y1 = 300
Population of S2 in Y1 = 600
⇒ Ratio = 300 : 600 = 1 : 2
$\therefore$ Statement I is wrong
Statement II: The population of S1 is 140% of the population of S2 in the year Y3.
From the graph,
Population of S1 in Y3 = 350
Population of S2 in Y3 = 250
⇒ Required percentage = $\frac{350}{250}\times 100=140$%
$\therefore$ Statement II is correct
Hence, the correct answer is 'Only II'.