Step 1: Understanding the Concept:
This question tests knowledge of prepositions and phrasal verbs. The verb "decide" can be followed by different prepositions, but the meaning changes.
Step 2: Detailed Explanation:
- decide on/upon something: This means to choose something from a number of possibilities.
- decide about something: This means to think about something in order to make a decision.
- decide in and decide of are not standard phrasal verbs with "decide".
In the given sentence, the context implies that they made a choice to select the white car among other potential cars. Therefore, "decided on" is the correct phrasal verb.
The complete sentence is: "They decided on the white car."
Step 3: Final Answer:
The correct option is (B) because "decide on" means to choose or select.