Final answer:
Pork is the most frequently prohibited food across various cultures, largely due to religious beliefs found in Judaism and Islam, as well as in some cultural groups that may not necessarily adhere to these religions.
Step-by-step explanation:
The food that is the most frequently prohibited across cultures is pork (a). This prohibition is often rooted in religious beliefs, such as in Judaism with kashrut laws, which forbid the consumption of pork, and in Islam, where pork is also not allowed. The cultural tradition of not eating pork extends to other groups that may not strictly be following religious dietary laws, but have cultural reasons for avoiding this food. In contrast to other options like beef, which is mostly prohibited in cultures like Hinduism, or shellfish, which is restricted primarily in Judaism and some branches of Christianity, the prohibition of pork is more widespread.
Similarly, other foods such as fish can be found as prohibited among certain East African groups, yet this is not as common as the prohibition against pork. Additionally, these food prohibitions can sometimes be temporary as in the case of Christian observance of Lent, where certain foods or drinks are given up as symbolic sacrifice.