'Net metering' is sometimes seen in the news in the context of promoting the

1. A

   Production and use of solar energy by the households/consumers

   Household solar production and use. Net metering credits a consumer for surplus rooftop solar (or other DG) exported to the grid against later imports—the policy tool that made small solar viable. That stored letter is the official key.

2. B

   Use of piped natural gas in the kitchens of households

   Piped natural gas in kitchens. PNG billing is a gas-utility meter, not net metering. This letter is not the key.

3. C

   Installation of CNG kits in motor-cars

   CNG kits in cars. Vehicle conversion is unrelated to bidirectional electricity metering. This letter is not the key.

4. D

   Installation of water meters in urban households

   Urban water meters. Water metering is not called net metering in the power-solar sense. This letter is not the key.

Summary. Official key is (a) production and use of solar energy by households/consumers. Net metering is a two-way electricity-account for rooftop solar. PNG, CNG kits and water meters are different. Honour the stored letter (a).