Reply with what the notice lists
Section 142(1) is how the officer asks for a return that was not filed, or for accounts and documents, before an assessment is finished. It is an inquiry. The notice is the list. Work down that list. A general letter that does not touch the points does not answer it.
File the reply in e-proceedings, from the registered e-filing account. Pending Actions is the way in. Keep a copy of what you uploaded and the acknowledgement the portal gives.
If the notice is not in the account, check the document identification number on the SMS and email page and raise a grievance if the PDF and the account do not match. Do not build a pack for a communication you cannot find.
Send the list on the notice, and we will say which papers answer it and which date is the one that binds.
Where the money came from
When the question is the source of a credit, send the statement that shows the credit and the paper that shows where it started. For money that came into India, that is the bank statement and the inward remittance advice. For a sale, it is the agreement and the receipt. For salary or a pension paid abroad, it is the statement that shows the credit was not Indian income of that year, if that is the fact.
Send what the notice names. If it names a year, send that year. A thicker pack of unrelated accounts does not answer a narrow question, and it is harder to read.
The same theme on a property purchase is on the section 148A page. A 142(1) is earlier than that show-cause. Do not wait for the later notice to explain the credit.
Asking for more time
The notice states the date. There is no general extra period stated on this page, because the statute does not hand you one in the text relied on here. If you cannot meet the date, write that in the e-proceedings reply, say what you already have, and ask for time. Do not assume the date has moved until the portal or the officer says so.
If the notice is a direction to file a return, the reply is the return, not only a letter. The case where no return was filed is on the non-filer page. Leaving a notice unanswered is what that page treats as the expensive path. This page does not restate a penalty figure for silence.