Question

what helped spain become one of the richest and most powerful countries in europe a:encomiendas b:learning new skills c: new colonist :d silver mines

Answer 1

Spain for over 300 years became into a very wealthy and powerful empire, since Cristopher Columbus arrived to America until 19th century. This was due to their colonies, from which they got access to many resources, such as gold, silver and many other goods.

This is why the answer is C, because the encomiendas and silver mines, or better: the access to them, were a consequence of the colonial process.