Some part of the given sentence is underlined.Four choices for rephrasing the underlined part follow each sentence.Please tell me which option is grammatically correct and effective.

The public library allocated revenues it received from the recently passed tax initiative to the purchasing of more than 2000 books and 50 computers.

(a) to the purchasing of more than 2000 books and 50 computers. See my comment below on B. There's nothing wrong with this option (A), although the noun form 'purchase' instead of the gerund 'purchasing' sounds more natural.
(b) in order to purchase more than 2000 books and 50 computers. Normally, when one speaks of allocating, one says what the money was allocated to/for. 'In order to' focuses on why they were allocating. I wouldn't say it's wrong, but it's less natural. 
(c) so as to purchase more than 2000 books and 50 computers. Same comment as B.
(d) for the purchase of more than 2000 books and 50 computers.See my comment above about B. This option (D) is fine and sounds natural. This is most likely the answer that the examiner is expectiong.

Please let me know which option is correct and why are the others incorrect.

Best wishes, Clive