Concept:A later-comer who arrives after the prayer has begun must join the congregation at once, without waiting for the next rak'ah.
Explanation:When a worshipper enters the mosque and finds the imam already leading the prayer, he should join the congregation immediately in whatever position the imam is in.
Waiting for the congregation to finish or praying alone is not correct for an able person who intends to pray in congregation.
After the imam completes the prayer and gives salutations, the later-comer then stands up and completes the number of rak'ahs he missed.
This practice is based on the teaching of the Prophet (SAW) that whatever you catch with the imam is the beginning of your prayer.
Therefore, the later-comer should not delay or observe his own prayer separately while the congregation is still in progress.
Answer:A later-corner to a congregational prayer should join the congregation immediately. Hence, the correct option is A.