The best and the most reliable – though it costs $100 is an E-Tourist visa. You can apply for an E-Tourist visa at https://indianvisaonline.gov.
There are many fake websites pretending to be the government site but charging additional fees. The above one is considered the best link by a friend from the UK who has spent many years in India and still visits India, at least once a year. The E-Tourist visa gives 3 months and multiple entry so is good considering it allows multiple entry. For those just passing through and only going into Delhi for one month, one could think of a Transit visa.
Even if you are not leaving the airport, you will probably need to clear immigration in order to collect luggage and re check for the flight to Paro Intenational Airport in Bhutan, so in this case, you will still need a visa.
You should also check with your local embassies or consulates about transit visa possibilities. If you are told you can get a transit visa on arrival, you should still have a WRITTEN PROOF before leaving home.
We recommend everyone to process an E-Tourist visa. The process online is a bit messy and frustrating (Hello India!) but it works out in the end. Visas take up to 72 hours to clear.