The best pull factor as an example is the safety which can lead towards immigration.
Answer: Option D
Explanation:
The safety is best promising scenario which establishes itself as the best pull factor. Pull factor for immigration of people inviting into a nation or state are based on better promising condition of jobs, livelihood, happiness indices or similar key factors, these all in a way contribute to safety.
Safety towards the living indices, better employment, plenty of resources, good management, religious stability with freedom of speech and expression, gender equality or such things, causes the effect of safety and hence immigration can be achieved. Other options mentioned are push factors.