The abc reports that more than 50% of the US population consumes meat,

The abc reports that more than 50% of the US population consumes meat, significantly more than earlier decades.

Explanation:

a) earlier decades
Illogical comparison of population with decades

b) earlier decades did
Illogical comparison of population with decades

c) it did in earlier decades
Correct.Parallel and logical comparison of action of 50% of population with action of population in earlier decades.

d) in earlier decades
Comparison element is missing.Action of Meat eating population is compared with nothing

e) that of earlier decades.
Incorrect.We need an action verb to complete the comparison.

Hence C is the correct answer IMO

The abc reports that more than 50% of the US population consumes meat, significantly more than earlier decades.

ABC reports that most of the US population consumes meat and __________ was significantly more than that of earlier decades.
What can be _______?
A) Percentage of US population,
B) meat,
C) the consumption of meat
D) the population itself
E) None
1) >50% of the US population consumes meat and this percentage is significantly more than that of earlier decades.
2) >50% of the US population consumes meat and meat was significantly more than that of earlier decades. So you mean to say they are consuming more meat now?
3) >50% of the US population consumes meat and the consumption of meat was significantly more than that of earlier decades.
4) >50% of the US population consumes meat and the population was significantly more that that of earlier decades. So you mean population has been increased?

All the options can make sense therefore it becomes very important to understand what exactly the author wants to compare?

a) earlier decades ------------Here decades are compared with what? population?consumption of meat? Whatever it may be it can't be compared with decades.

b) earlier decades did
--Same reason as A

c) it did in earlier decades
--Here it refers to population. Yes population can be singular/plural and it all depends on the meaning.
The inferred meaning would be:
The abc reports that more than 50% of the US population consumes meat, significantly more than the population did in earlier decades. Makes sense.

d) in earlier decades
--significantly more than in earlier decades?...what in earlier decades?

e) that of earlier decades.
--This was an interesting option but as mentioned before 'that of earlier decades' can be anything? Population, meat, consumption etc. So there is an ambiguity. Hence it's incorrect.