21) D

Out of all the options, D is the correct one.

A – Wrong
In 1990s, total number of crashes = 235
Crashes due to pilot error = 140.
Proportion = 140/235 ≈ 0.6
You can see that in any decade, proportion of crashes due to pilot error is approximately 0.6
So proportion of crashes due to pilot error was not unusually high in the 1990s.

B – Wrong
In 2010s, total number of crashes = 66
Crashes due to mechanical error = 11.
Proportion = 11/66 ≈ 0.17
You can see that in any decade, proportion of crashes due to pilot error is approximately 0.17
So proportion of crashes due to mechanical error was not unusually low in the 2010s.

C – Wrong
Proportion of crashes due to bad weather in 1960s = 14/248 ≈ 0.06
Proportion of crashes due to bad weather in 1970s = 13/238 ≈ 0.06
Proportion of crashes due to bad weather in 1980s = 11/205 ≈ 0.05
Proportion of crashes due to bad weather in 1990s = 13/235 ≈ 0.06
Proportion of crashes due to bad weather in 2000s = 7/112 ≈ 0.06
Proportion of crashes due to bad weather in 2010s = 5/66 ≈ 0.08
So proportion of crashes due to bad weather did not steadily decrease over the period.

D – Correct
Proportion of crashes due to sabotage in 1960s = 12/248 ≈ 0.05
Proportion of crashes due to sabotage in 1970s = 25/238 ≈ 0.10
Proportion of crashes due to sabotage in 1980s = 23/203 ≈ 0.11
Proportion of crashes due to sabotage in 1990s = 19/235 ≈ 0.08
Proportion of crashes due to sabotage in 2000s = 10/112 ≈ 0.09
Proportion of crashes due to sabotage in 2010s = 6/66 ≈ 0.09
So proportion of crashes due to sabotage was unusually low in the 1960s.

E – Wrong
If you work out the estimate proportions, you will realise that the proportions of crashes due to other causes increased steadily over the period.