Explanation:

Correct Option is 1: are on the field

The construction as well as is used to conjoin two expressions, but it cannot be used in the way that we use and. It is used to add an expression with the first one but not to combine them together.
For example, we say: 'My mother and father are coming to the party. (The subject becomes plural)
But when we use as well as we say: 'My mother, as well as my father, is coming to the party.' (The subject my mother remains singular)
Therefore, in the given sentence, it is inaccurate to use the plural verb form are since the subject of the verb is the captain i.e. singular.
Hence, the error is in the part of the sentence in option 1.