The correct option is B Were
The auxiliary verb 'be' has many forms. Depending on the person (first
person, second, third), the number (singular or plural) and the tense (past: was, present: is, future: will be) the form is
used.
'To be' means to exist or live.
In the given sentence, the
subject "Several people" is in the third person plural narrative.
From
the options, 'were' is the most suitable verb. This is because 'were' is a
form of 'be' used for the third person plural narrative. Hence B is
correct.
Option A: 'Was' is similar to 'were' but the difference is that it's used for a singular subject. 'People' is plural, hence A is incorrect.
Options C and D: The auxiliary 'had' means 'experience or undergo'. For example, 'I had a surgery last week.' 'Have' is a form of 'had'; the former is in the present tense whereas the latter is in the past tense.
These verbs do not agree with the sentence in terms of form. If either of them were to be used, the agreeable forms would be 'had been' or 'have been'.
Hence C and D are incorrect.