Minimum number of NOR Gates required are 3.
Note:- Minterm and Product term are different.
Every Minterm can be Product term but not vice versa.
For example, in a 4 variable function, term "BD" is product term but not minterm. Because, Minterms are terms that should contain every variable either in true or complemented form. The term "BD" doesn't contain both A and C either in true or complemented term.
Whereas the term "ABCD" is a minterm for a 4 variable function and it is equal to 15.
Let's come to the question now. The function is expressed in sum of minterms. Now we have to minimize it. We can minimize it into either SOP or POS as there are no restrictions on the questions.
So, using 3 NOR gate, we can minimize.