India lost 11.73 lakh man days to strikes in 2017, and according to estimates made by the Labor Bureau, they cost the nation 550 crore rupees.

A. to strikes in 2017, and according to estimates made by the Labor Bureau, they cost the nation 550 crore rupees -- What is the reverent for ‘they? Strikes, man-days, or estimates? Wrong

B. to strikes in 2017, which according to estimates made by the Labor Bureau, cost the nation 550 crore rupees -- The correct choice.

C. in strikes in 2017, and according to estimates made by the Labor Bureau, it cost the nation cost the nation 550 crore rupees --- 1. referent for ‘it’? Wrong 2. anything wrong with the transcription?

D. in strikes in 2017, so as to cost the nation 550 crore rupees, according to estimates by the labor bureau --- 'so as to cost' is a wrong idiom.

E. to strikes in 2017, which according to estimates made by the Labor Bureau, thus costs the nation 550 crore rupees --- the plural subject, whether man-days or estimates or strikes does not have a plural verb
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I am going to approach this question exactly how I would approach any SC questions in GMAT. The moment we encounter one definite problem with one option, we will reject it.

We first notice have 2 splits -
India lost 11.73 lakh man days to strikes
India lost 11.73 lakh man days in strikes

Now the later usage is definitely wrong idiomatic usage, so immediately we C,D out. We are left with A,B and E

Now lets come to another split, the pronoun usage
by the Labor Bureau, they cost the nation
by the Labor Bureau, cost the nation

The first one has "they" which should logically refer to strikes, but it is actually referring to the Labour Bureau. So A is out. Since B and E have no such pronouns we keep them in.

Now let us zoom at E.
thus costs the nation 550 crore rupees

the verb "costs" is singular. the subject here is "strikes" is plural. We have a SV agreement problem.

Answer: B

India lost 11.73 lakh man days to strikes in 2017, and according to estimates made by the Labor Bureau, they cost the nation 550 crore rupees.

A. to strikes in 2017, and according to estimates made by the Labor Bureau, they cost the nation 550 crore rupees

B. to strikes in 2017, which according to estimates made by the Labor Bureau, cost the nation 550 crore rupees

C. in strikes in 2017, and according to estimates made by the Labor Bureau, it cost the nation cost the nation 550 crore rupees

D. in strikes in 2017, so as to cost the nation 550 crore rupees, according to estimates by the labor bureau

E. to strikes in 2017, which according to estimates made by the Labor Bureau, thus costs the nation 550 crore rupees

daagh :- "in 2017" is an adverbial modifier and thus ", which" can not jump over this to modify lost man days. Isn't that right?
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Shamkeev

I do not go with your rationale. The relative pronoun should not jump over an actual verb, but I do not think that it cannot jump over another inessential modifier.
But the point is why you want to go into a modification when there is an easier error to spot, i.e., the SV error. Such a deviant mindset will be a great drain in the Hall.

Even the correct choice B, one has to take with a pinch of salt because what the cost the nation so much of money was the loss of man-days and you just don’t have the word ‘the loss’ in the choice. Therefore, just give the devil its due and be done with.
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India lost 11.73 lakh man days to strikes in 2017, and according to estimates made by the Labor Bureau, they cost the nation 550 crore rupees.

A. to strikes in 2017, and according to estimates made by the Labor Bureau, they cost the nation 550 crore rupees -- ambiguous pronoun "they" could refer to man days OR estimates

B. to strikes in 2017, which according to estimates made by the Labor Bureau, cost the nation 550 crore rupees -- "which" refers correctly to man days and SVA is correct too.

C. in strikes in 2017, and according to estimates made by the Labor Bureau, it cost the nation cost the nation 550 crore rupees -- "it cost" wrong SVA

D. in strikes in 2017, so as to cost the nation 550 crore rupees, according to estimates by the labor bureau -- "so as to" makes it seem like intentional cause-effect between lost man-days and cost to the nation.

E. to strikes in 2017, which according to estimates made by the Labor Bureau, thus costs the nation 550 crore rupees -- grammatically wrong. "thus" is incorrectly used for conjunction

Answer: B

May someone explain why "in" strikes is wrong?
Many thanks in advance

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In option B, phrase "according to estimates made by the Labor Bureau" needs to come under comma pair, otherwise "which" doesnt pair up with the verb "cost".
So just after "which" we need a comma.

In would essentially refer to the position or place. eg - I am in the middle of a war. However, there are other issues going on here even if ignore the idiom.