Acting as a 接続詞（せつぞくし） (conjunction), それなら is a structure used for highlighting that if the previous statement is the case, then (B) will probably also be true, or will need to be considered. それなら may be interpreted as ‘if that’s the case, then (B)’.

• 駅（えき）の近（ちか）くにあるスーパーに行（い）くの？それならついでに私（わたし）を駅（えき）まで送（おく）ってくれない？
  You are going to the supermarket near the station? If that’s the case, can you drop me to the station?
• カメラの画質（がしつ）がいいスマホをお探（さが）しなんですか？それならこのスマホをお勧（すす）めします。
  You are looking for a smartphone with a good camera? If that’s the case, I recommend this one.

それなら is most often used when replying to some statement that the speaker has just heard from another person. In these cases, it will come before some sort of guess, suggestion, or consideration about what the speaker has just heard.

• それなら彼（かれ）でもできるようだ。
  If that’s the case, it seems like even he will be able to do it.
• それなら早（はや）く警察（けいさつ）に電話（でんわ）したほうがいいよ！
  If that’s the case, you should call the cops as soon as possible.
• それなら先生（せんせい）に聞（き）いたほうがいいと思（あも）います。
  If that’s the case, you should ask the teacher.

だったら and それだったら may also be seen, and carry the same meaning.

• 一人（ひとり）で食（た）べに行（い）くの？だったら僕（ぼく）も一緒（いっしょ）に行（い）ってもいい？
  You are going to go eat alone? If that’s the case, can I come with you?
• 田中（たなか）君（くん）もコストコに行（い）くの？それだったら一緒（いっしょ）に行（い）こうよ。そのほうがガソリン代（だい）も節約（せつやく）できるし。
  Tanaka-kun, are you also going to Costco? If that’s the case, let’s go together. That way, we can save money on gas.