There is some unknown number s of subsequent tosses; the total number of heads is 6 + s; the total number of tosses is 12 + s.

Statement (1) is insufficient. It could be that s = 12. In that case, the last four of the first 12 tosses were all tails, and the total number of tosses is 24. It's also possible that s = 6. If the first 12 consisted of, in order, 6 tails and 6 heads, then followed by 6 more heads, we're still following the rule in this statement, but the total number of tosses is only 18.

Statement (2) is sufficient. We can express the percent of total tosses that resulted in heads as (6 + s)/(12 + s), and given this statement, we can set that equal to 3/4:
(6 + s)/(12 + s) = 3/4
4(6 + s) = 3(12 + s)
24 + 4s = 36 + 3s
s = 12

The total number of tosses is 12 + s = 24. Choice (B) is correct.