A "learning rate" expresses the ratio of the labor spent on the n-th item to that spent on the (n/2)-th item. That is, the 8th helicopter manufactured took days related by ...
0.8 × (days for 8th) = (days for 16th)
(days for 8th) = (days for 16th)/0.8 = 1.25×(days for 16th)
Then the days for the 4th are 1.25 times that, and the days for the 2nd are 1.25 times the days for the 4th. The days for the first helicopter will be
... 1.25⁴×106.496 days = 260 days
The appropriate choice is
... d. 260
