Hola Shirley

We are using the subjunctive. But in English it wouldn't work if we said "I hope I had a son by the time I'm 33", because we are talking here about a future event, so we have to say it in the present "I hope I have..." 

In Spanish, you can either use the present subjunctive or the imperfect subjunctive (ojalá tenga...- ojalá tuviera); the imperfect subjunctive would create the idea of "less probability".

We are testing conjugation here so we can only admit "tuviera" -the hint is asking for conjugation of the imperfect subjunctive of tener.

I hope this clarified it.

Saludos

Inma