Questions

Fossil fuels energy source generates the least greenhouse gases. Of them, i.e, oil, natural gas and coal, natural gas source generates the least greenhouse gases.

The capital of India shift from Kolkata to New Delhi in the year 1911.

Let the missing numbers be x, y, a and b

x + y = 8 ---- (1)
a - b = 6 ---- (2)
x + a = 13 --- (3)
y + b = 8 ---- (4)

2x + a + y = 21 --- From (1 & 3)
a + y = 14 ----  From (2 & 4)

Substitute (2 & 4) in (1 & 3)

2x + 14 = 21
2x = 7
x = 3.5

From (1) ---> 3.5 + y = 8
=> y = 8 - 3.5 = 4.5

a + y = 14 => a = 14 - y = 14 - 4.5 => a = 9.5
From (4) ---- y + b = 8 => b = 8 - y = 8 - 4.5 = 3.5

Hence,
x = 3.5
y = 4.5
a = 9.5
b = 3.5

The meanings of the given words are as follows:
Histrionic: excessively theatrical or dramatic in character or style
Melodramatic: characteristic of melodrama, especially in being exaggerated or overemotional
Theatrical: exaggerated and excessively dramatic
Conscientious: wishing to do one's work or duty well and thoroughly
Diligent: having or showing care and conscientiousness in one's work or duties
Tenacious: tending to keep a firm hold of something; clinging or adhering closely
Retentive: (of a person's memory) effective in retaining facts and impressions
Thrive: (of a child, animal, or plant) grow or develop well or vigorously
Tussle: a vigorous struggle or scuffle, typically in order to obtain or achieve something
Pertinent: relevant or applicable to a
particular matter; apposite Appropriate: suitable or proper in the circumstances
Therefore, it can be observed that option  i.e. ‘melodramatic, theatrical’ forms a synonym- synonym pair of ‘histrionic’.

The difference between Greenwich Mean Time & Indian Standard Time is 5 1/2 hours.