Why weren't slave ships allowed to dock at port?

Due to the fear that they might've spread diseases.

Explanation:

There's a widespread perception among Europeans at that time that African people have a higher tendency to carry diseases that's not many doctors in their country familiar with.  So , they never let slave ships to dock at a port. They're afraid of the "contamination".

Their suspicion often supported with the fact that slave ships usually smell really bad. They believed it's something related to the disease.  But that's not case. Slaves were chained down to the floor for months during the journey and forced by the slave traders to "use the toilets" right where they sleep. It was horrible. That's where the smell came from and People at that time were very ignorant about this fact.

i think its provide for the common defense

the answer is option a.