Final answer:
The river that joins the Jarama in Spain and flows into Portugal is the Tagus River, option c.
Step-by-step explanation:
The river that joins the Jarama River at the 40th parallel north and flows into Portugal is the Tagus River. Unlike the Nile River, which is known for its contribution to the development of the Egyptian civilization, or the Danube and the Rhine, which are crucial European rivers playing a vital role in transportation and trade, the Tagus River specifically crosses from Central Spain into Portugal, ending at the Atlantic Ocean.