Hello, I have a Macbook pro model number A1278 - which power adapter do I need? Thanks

You have a 13" MacBook Pro non-Retina from 2009-2012 which uses a Magsafe 1 charger. The 13" laptops came with a 60W charger, but you can use the 85W charger as well. Here is the link to the US Apple Store for the Magsafe 1 charger (both 85W and 60W are selectable from the drop down list):

Apple 85W MagSafe Power Adapter - Apple

The 45W charger option is only meant for the MacBook Air and is unable to charge the battery while also using the laptop. You need at least the 60W charger.

Here is an Apple article to help users identify the correct charger for their laptops (you can select a larger charger, but never select one smaller than what is recommended for a particular model...just make sure to if you need Magsafe 1 that you get Magsafe 1 because the connectors for the Magsafe 2 chargers are physically different):

Identify your Mac power adapter - Apple Support

The number A1278 was uss for four years but all variants show using Apple 60W Magsafe 2 charger:

https://www.apple.com/shop/product/MC461LL/A/apple-60w-magsafe-power-adapter-for-macbook-and-13-inch-macbook-pro?fnode=80151776274a51ecc05f7454b8ac0effa12f9210f8d58b0704ecb7828a20c335676f6df94d09e882023b5391825eb73ba233e8598c712f0affbd28324c0372e99948a240a585529154cc9f5007cbc796981c26d0aecf740e92e828ed4155760958e293a7e423327d661be4628d4ec631&fs=f%3Dadapter-charger%26fh%3D4595%252B45b0%252B45d4

Allan Jones wrote:

The number A1278 was uss for four years but all variants show using Apple 60W Magsafe 2 charger:

FYI, you provided the proper link, but mentioned Magsafe 2. A 2009 to 2012 non-Retina model uses a Magsafe 1 charger.