(b) Discuss possible sources of bias in this study. Do you think it is possible to avoid all selection and measurement bias?

A possible source of measurement bias could be whether all the bolts were counted, which could lead to an underestimate of our total.

Also, a possible source of selection bias could be whether the sampled sections were truly representative of the strata for which they were chosen.

No. it is probably not possible to eliminate or to avoid all selection and measurement bias, particularly in field work as described here.