Select the most appropriate option to fill in the blank.

They sent us photographs _________ their baby. 

The correct answer is 'of'.

 Key Points

• The preposition 'of' is used to show possession or belonging.
• In the context of the sentence, the photographs belong to "their baby", thus 'of' is the most suitable preposition.
• The sentence aims to convey that the photographs depict their baby, making 'of' the correct choice.
• In English grammar, when showing what a picture is about, we use 'of'.

Therefore, the correct answer is Option 3.

Additional Information

• Option 1 ('to') would indicate direction, which doesn’t fit here.
• Option 2 ('on') would suggest that the photographs are located on top of the baby, which doesn’t make sense.
• Option 4 ('for') would imply that the photographs were intended for the baby, which is not the intended meaning.