The correct option is
A to accept
In order to fill in the blank with the most suitable verb, we need to understand the usage of 'make'. 'Make' means to force someone to do something. If 'make' 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 passive voice, we will not use the bare infinitive here; we will use the verb with the marker 'to'.
Option B - 'Accept' is a verb in its original form and is a bare infinitive. A bare infinitive is used in case of active voice, and as the given sentence is in the passive voice, we can't use 'to accept' here. Hence, option B is incorrect.
Option C - 'Accepts' is the third person form of the verb 'accept'. As explained above, we need the verb (infinitive) with the marker 'to'. Hence, option C is incorrect.
Option D - 'Accepting' is the participle form of the verb 'accept'. As we need the verb in its original form and with the marker 'to', we can't use 'accepting' here. Hence, option D is incorrect.
Option A - 'To accept' can be used here, as the infinitive is used with the marker 'to', and it is suitable in the case of passive voice. Hence, option A is the correct answer.
'The politician was made to accept his mistakes.'