I think it's option A, or number 2
<span>The correct option is B. Mike joined the Army, and he became more disciplined during the training. The correct way to connect these two clauses is to introduce a conjunction "and." A is incorrect because there should be a comma/conjunction between the words Army and he. C is incorrect because that semi colon is just randomly and incorrectly placed without a context. D is incorrect because there should also be the word "and" before "he."</span>
Explanation:
while, both, only one, however
ᴘᴇʀᴘᴇᴛᴜᴀʟ ɪ ʙᴇʟɪᴇᴠᴇ ʙᴜᴛ ɪᴍ ɴᴏᴛ sᴏ sᴜʀᴇ
