Answer:
The car that stopped running today was reliable for over 20 years.
Step-by-step explanation:
A relative clause is a group of words with a subject and a verb that tells us more about a noun, that is, acting as an adjective. These relative clauses can be defining (when it gives essential information about a noun) or non-defining (when it does not give essential information about a noun and is set off by commas).
The sentence “The car that stopped running today was reliable for over 20 years” is the one that combines the two sentences and includes a relative clause, which it's “that stopped running today.” This clause gives us essential information about the car (a noun), therefore it is a defining relative clause.