You said that this graph,

represents (a) b and d are density dependent.

yes -- compare the b value for an individual in a population with N = 100 with b for an individual in a population with N = 500. If it's different, then b depends on density. The same for d ... in this example, both b values and d values change between small and large populations.
Remember, b and d are the birth and death rates per individual.
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