The correct option is B was
Options B and D:
The words 'she _____ ready to leave' refer to a simple past action. We use the simple past tense verb 'was' to describe an action that was completed in the past.
The given sentence contains two actions, one that happened after another.
'Had been' describes a past perfect action; this tense describes an action 'When Sandhya _____ all the pictures' that was completed just before another commenced 'she _____ ready to leave'.
As the rule goes, the previous action is in the perfect tense and the one that follows is in the simple. Hence the correct verb here is 'was' and not 'had been'.
Hence option B is correct and D is incorrect.
Option A: 'Were' is a plural verb whereas the subject here is singular.
The verb must agree with the subject in terms of person and number. So, when the subject is singular, the verb must also be singular.
Hence A is incorrect.
Option C: 'Been' is a past participle and requires a helping verb to describe a tense.
There isn't any other verb in the sentence, hence C is also incorrect.