Elles achètent un cadeau_leur père.

In the sentence "Elles achètent un cadeau_leur père," we need to choose the correct preposition that fits into the blank. The sentence translates to "They are buying a present_for their father." The key is to look for a preposition that appropriately indicates the recipient of the gift.

The French word "pour" is used to indicate the purpose or the recipient of an action. In this context, buying a gift for someone means they are the intended recipient. Therefore, the correct preposition to use is "pour."

So, the completed sentence is: "Elles achètent un cadeau pour leur père."

To understand why the other options do not fit:

• "parmi" means "amongst," and it would not indicate the recipient of the gift.
• "dans" means "in," which refers to location and not a recipient.
• "entre" means "between," and indicates something in relation to two entities, not a recipient.

Thus, the only preposition that fits the meaning of buying a gift for someone is "pour."