Step 1: Look at each country in the group.
The Southern Cone includes Argentina, Chile, Paraguay, Uruguay and Peru. All five nations were colonized by Spain, not Portugal, France or Britain.
Step 2: Check the colonial history.
Spain ruled most of South America from the 1500s onward, and Spanish became the official and everyday language in these territories. Brazil is the one big exception, since Portugal colonized it and Portuguese is spoken there instead.
Step 3: Rule out the wrong options.
Portuguese is Brazil's language, not this group's, so option A fails. French is spoken in French Guiana, a small territory outside the Southern Cone, so option C fails. English has no colonial hold in this region at all, so option D fails.
Final Answer:
Spanish is the common, dominant language across Argentina, Chile, Paraguay, Uruguay and Peru. \[ \boxed{\text{Spanish}} \]