*Presumably*, given your title and the implicit assumption when things are stated this way,

by the vector space , you mean the vector space over the base field

For (i) maybe this is a case where abstraction clarifies rather than confuses? Maybe it removes the the jumble formed by the specific values?

Suppose I tell you I have a vector space V over a field F. Suppose I give you a fixed vector in V, and ask you to find a basis and the dimension of the subspace formed by all F-multiples of that vector:

You do everything *exactly* as you would for a real vector space, except now the coefficients are allowed to be complex numbers. That goes not just for problem (i), but for all 5 problems. Ask yourself, "How would I do the same problem, except for a real vctor space using real coefficients?" If you can answer that, then you should be able to solve these. Every procedure is *exactly* the same, except that here it uses complex coefficients.