a and b are correct

for left and right hand sums the error bound is

En =[ K(b-a)^2]/2n

where K is the max of the first derivative and n is the number of sub intervals

K =13

13(1.5)^2/(2n) < .1

solving this inequality we get n = 147 ( to the nearest integer)

which means he would have to take readings about every .6 min or every 36 secs.