A part of the sentence is underlined. Below are given four alternatives which may improve the word. Choose the correct alternative.
I hadn't been seeing him for the last 10 years.

Verbs of perception such as see, taste, smell, hear, prefer, notice, please, recognize etc. do not take 'ing' form.
And hence do not come in the continuous tense.
Thus, to convert the sentence to Past Perfect tense instead of Past Perfect Continuous tense, in order to make it grammatically correct, we will have to replace the underlined words with 'seen'.
Hence, option 2 is the most appropriate choice.