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(a) was having a headache
This option doesn't work because "was having a headache" is in the past continuous tense, which doesn't match the tense of the rest of the sentence. The sentence begins in the past perfect continuous tense ("I had been having a headache"), but then abruptly switches to the simple future tense ("I will not attend the meeting"). Therefore, this option doesn't provide the necessary consistency in verb tense.
(b) have a headache
This option provides the correct verb tense and structure to maintain grammatical consistency with the rest of the sentence. The phrase "have a headache" is in the present tense, which matches the present perfect continuous tense of the first part of the sentence ("I had been having a headache"). "Have a headache" accurately conveys the idea that the speaker currently has a headache, which aligns with the context of the sentence indicating a present condition affecting the speaker's ability to attend the meeting. This option is grammatically correct and idiomatic, making it the most suitable alternative to complete the sentence accurately.
(c) used to have a headache
This option doesn't fit the context of the sentence. "Used to have a headache" typically implies a past habitual action, but it doesn't convey the idea that the speaker currently has  a headache. It suggests that the speaker no longer experiences headaches, which contradicts the context of the sentence.
(d) have headache
This option is incorrect because "have headache" is not a grammatically correct phrase. In English, "headache" is typically used as a countable noun, so it requires an article ("a" or "the") or a possessive pronoun ("my," "your," etc.) before it. Therefore, "have a headache" or "have had a headache" would be correct, but "have headache" is not.