B
Explanation:
For a ruler with a 1 cm increment, the uncertainty for each reading is +/- 0.5 mm.
Thus, the uncertainty of the measurement, which is made up of two readings, is double the uncertainty of a single reading i.e. +/- 1 mm.
Hence, the length of the thread is (45.3cm – 12.6 cm) = 32.7 +/- 1 mm. Thus, the maximum possible length is 32.8 cm.