2.300+0.02017+0.02015=2.3432
Rule: The result of an addition or subtraction is reported to the same number of decimal places as present in number with the least decimal places.
Since the least precise number is 2.300 and that has three digits after the decimal, therefore the result of the above addition is rounded off to three decimal figures as 2.340.
126,000 has 3 significant figures as zeros are not counted
15.15μs=15.15×10−6 s=1.515×10−5 s
0.0048=4.8×10−3
Hence, the correct option is D.