(b.) I think the premise of this question is invalid or ambiguous. sizeof is not used to find the length of the vector, and it also technically isn't a function. vector.size() is what should be used to find the number of elements in a vector. If by "length", you mean the size in bytes of internal data on the heap, you can do my_vector.size() * sizeof(TypeOfMyVector), ex: my_vector.size() * sizeof(int).
For the other questions, you should put some more effort into trying to solve it yourself first, and then come back when you have a specific question.