Final answer:
The correct verb form is 'serían.' It is the conditional tense conjugation of the verb 'ser.'
Step-by-step explanation:
The correct verb form to complete the sentence is serían. 'Serían' is the conjugation of the verb 'ser' in the conditional tense for the third-person plural subject pronoun. It is used to indicate a hypothetical situation or an action that would happen if certain conditions were met.
For example:
Si fueran más generosos, serían menos tacaños. (If they were more generous, they would be less stingy.)
So, the correct sentence would be 'Serían demasiado tacaños.' indicating that 'they would be too stingy.'
Learn more about Spanish verb conjugation