The correct option is
C atttend
In order to fill in the blank with the most suitable form of the infinitive, we need to understand the usage of 'had better'. 'Had better' is similar to 'would rather', which means preferring something or someone over the other. If 'had better' is used in a passive (voice) sentence, we use the word 'to' along with the verb. In an active sentence, we do not use 'to' with the verb. The verbs without the marker 'to', are called bare infinitives.As the given sentence is in the active voice, we will use the bare infinitive here.
Option A - Bare infinitive is a verb in its original form. As 'attending' is the past participle form of the verb 'attend', we can't use it here. Hence, option A is incorrect.
Option C - 'Attends' is the third person form of the verb 'attend'. As we need the verb in its original form, we can't use 'attends' here. Hence, option C is incorrect.
Option D - 'To attend' can't be used here as the verb in its original form with the marker 'to'. As the sentence is in the active voice, we can't use the infinitive with 'to' here. Hence, option D is incorrect.
Option B - 'Attend' is a verb in its original form and is a bare infinitive. Hence, option B is the correct answer.
'We had better attend today's important meeting.'