Error Spotting:The father(a) / as well as the sons were(b) / mysteriously missing (c)/ from the house.(d)
a
b
c
d
Explanation:
Replace ``were`` with ``was``
When two subjects are joined by as well as the verb agrees with the first subject.
Above in the question the first subject is "The Father" which is a singular so we need to use first firm of the verb which is `was` instead of `were`.