The correct option is
B I am loving
The sentence has to be filled with either present continuous or perfect continuous form of the verb. Present continuous tense is used to define the actions that are continuing at the present moment. Present perfect continuous tense is used to define the actions happening for a long time and are unfinished. Here, the action doesn't seem to be happening for a long time, hence, we will use the present continuous tense (RULE: is/am/are + Verb+ing).
Option B is correct, as 'I am loving it' is the correct form of the present continuous tense.
Option A is incorrect, as 'I love it' is simple present tense and we can't use it here as the question demands us to use either present continuous or perfect continuous tense.
Option C is incorrect, as 'I loving' is grammatically incorrect. In case of present continuous tense, we need an auxiliary verb before the verb. Here, we need 'am' to complete the sentence.
Option D is incorrect, as 'I loved it' is simple past tense and we can't use it here as the question demands us to use either present continuous tense or perfect continuous tense.