Here, we have to fill in the third blank of the given informal letter. As the first half of the sentence is in present perfect tense, the second blank must be filled in the same tense as well.
Option A - 'Sending' is a verb in present continuous tense. 'That you have sending' is grammatically incorrect. Hence, option A is incorrect.
Option C - 'I have just received the lovely camera that you have seen for my birthday' is grammatically as well as logically incorrect. If Raj's father had seen a camera already, the sentence should be - 'had seen for my birthday'. As the sentence is 'that you have...', seen is a wrong choice. Hence, option C is incorrect.
Option D - 'Gift' is a verb as well as noun, and it is not in perfect tense. Hence, option D is incorrect.
Option B - 'Sent' is a verb in perfect tense. 'I have just received the lovely camera that you have sent me for my birthday' is grammatically and logically correct. Hence, option B is correct.