According to me, 3 is the number of Essential Prime Implicants.
This is how minimal expression is obtained.If a don't care is used in getting minimal solution, then the group with that don't care can also be considered as EPI(provided it is grouped only once). Here, don't care must be included to form a quad. That quad is necessary to form Minimal Expression.
Thus we have 5 Prime Implicants and 3 Essential Prime Implicants.
One of my classmates is saying that number of Essential Prime Implicants is 2. We can't include don't cares even if they are a part of minimal expression.
So, the number of Essential Prime Implicants is 2 or 3?