(a) Tanu, and also Anu, is preparing /(b) for CAT, but it

(a) Tanu, and also Anu, is preparing /(b) for CAT, but it is still /(c) not clear if that is the best option for them. /(d) No error

The subject is plural, Anu and Tanu, hence plural verb is required: 'Tanu and also Anu are preparing.'