Question

imperfect er/ir
yo\tía\tnosotros/as\tíamos
tú\tías\tvosotros/as\tíais
él/ella/
usted\tía\tellos/ellas/
ustedes\tían
¿______ (construir) con bloques mucho vosotros?
construíais
construíamos
construían

Explanation:

The question uses the subject "vosotros/as", which in the imperfect tense for ER/IR verbs takes the ending "-íais". The stem of "construir" (to build) in the imperfect is "construí-", so combining the stem and the correct ending gives the conjugated form.

Answer:

construíais