answering your question or in answering
Informally , when writing a letter in respond to another one , what do you most say'

Answering your question ,I think you should....

As you asked me , I think....

In reply to your question....

Thanks

Question

answering your question or in answering
Informally , when writing a letter in respond to another one , what do you most say'

Answering your question ,I think you should....

As you asked me , I think....

In reply to your question....

Thanks

Answer

To answer your question.....is least formal. At least to me it is. So it all depends on the situation. Hannah has supplied the most usual phrases.

Answer

In answer to your question... I would say:
"In response/answer to your question".  "In answer to..." sounds slightly less formal than "in response to...".  
"To answer your question..." could also be used.