Option A: 'Many of' refers to a great number of "my friends". It's the most appropriate option, as 'many' refers to things and people. Hence option A is correct.
Option B: 'Little' refers to a quantity of something, 'little milk', 'little food'. It does not refer to a number of people.
Option C: 'Mostly of' refers to a part in terms of a whole. It does not indicate the number of friends, in this context. For example, 'milk comprises mostly of water.'
Option D: 'Handful' requires an article (a handful) as well as a preposition (a handful of) to complete its meaning. Without these elements, the word isn't right here.
Hence options B, C and D are incorrect.