In 1981 children in the United States spent an average
of slightly less than two and a half hours a week doing
household chores; by 1997 they had spent nearly six
hours a week.
A. chores; by 1997 they had spent nearly six hours
a week
B. chores; by 1997 that figure had grown to nearly
six hours a week
C. chores, whereas nearly six hours a week were
spent in 1997
D. chores, compared with a figure of nearly six hours
a week in 1997
E. chores, that figure growing to nearly six hours a
week in 1997

OA is B. Please help me explain why B is correct?

well, as with many problems, your most successful approach to this one may be to figure out what's wrong with the 4 wrong choices, rather than what's right about the correct one.

choice a: the tense (past perfect) is weird; it implies that they had put in 6 hours of work (total) by 1997. alternatively, it implies that they had put in 6 hrslwk at some point prior to 1997, but that they hadn't maintained that level of output.

choice b is ok:
- past perfect makes sense, because the figure had finished growing by then. (note that the past perfect makes sense here, even though there isn't an explicit 'second event': in the present, we'd say the figure has grown to such-and-such a level, so, by 1997 [a past date], we'd say it had grown to such-and-such a level.
- the sentence uses the construction 'that figure', thus avoiding any weird pronoun/modifier references.

choice c: needless use of the passive voice. at best this is awkward, and at worst it makes the reader think that the 6 hours of chores were being performed by someone else (i.e., not children).

choice d: illogical - the chores themselves are 'compared with a figure of...' which doesn't make sense.

choice e: just plain bad grammar in the construction of the modifier, plus unacceptable change in meaning (this sentence, unlike the original, declares that the growth up to 6hrs/wk occurred in 1997)

also in A the use of 'they' for children indicates that in 1997 and in 1981 the children are same. I feel it awkward. Ron please comment.

Hi Ron,

Thanks for your information.

I have question on the semicolon. Generally, semicolon is used to coordinate two independent clauses. In the above one “by 1997 that figure had grown to nearly six hours a week “ can stand on its own?

Thanks,

that's a good point; the use of 'they' does seem to suggest that the antecedent is invariant (that it's the same 'they' from '81). i hadn't thought of that.
well played.

correct.

here's a more substantial sentence that has the same structure (i.e., having a prepositional phrase as a leading modifier)
by the end of 1923 the german mark had depreciated so much that it was more valuable as kindling than as currency.
if you understand why that sentence is ok, then you also understand why the clause in question is a standalone sentence.