Which of the following is a run-on sentence? (a) a lion can run faster than a greyhound or a fox. (b) bald eagles build nests of branches; the next may weigh tons. (c) humpback whales sing songs; they sing new tunes every year. (d) some tortoises are huge, you can ride on them.

D is the run-on. This is an example of a comma splice. “Some tortoises are huge” and “you can ride on them” are both independent clauses, complete thoughts. If a period or semicolon had been used, as in the other choices, this would have been an acceptable sentence. However, only a comma is used.

C. it sounds the most correct when said out loud. sorry if i cant . 

She either must have not been on the phone and working or something else. it seems that here boss catches her a lot of the time on the phone and not doing work but hasn't been fired yet.