The correct answer is A. tengo. The direct translation of this sentence is 'I have 40 years', though it means 'I am 40 years old'. Since the subject of the sentence is 'Yo', or 'I', the verb needs to be conjugated to fit 'Yo'. The conjugations of the verb 'tener' are as follows:
Yo - tengo
Tú - tienes
Él, ella, usted - tiene
Éllos, ellas, ustedes - tienen
Nosotros/nosotras - tenemos
Vosotros/vosotras - tenéis
Therefore, we should use the conjugation 'tengo', since the subject of the sentence is 'Yo'.
Hope this helps!
