An execution occurrence specification in UML sequence diagrams is represented by a thin rectangle on a lifeline. It shows that an instance is performing an action over a period of time.

In the provided sequence diagram, we can see three such thin rectangles on lifeline 'b', which indicates that there are three execution occurrence specifications.

One for the message that comes from 'a' to 'b'.

Another one for the message that 'b' sends to 'c'.

And the last one for the return message from 'c' to 'b'.

There are no execution occurrence specifications on lifelines 'a' and 'c'.

Therefore, the correct answer is:

D. 3