i7-6700.

https://www.userbenchmark.com/UserRun/46585455

Thanks for replying! and there is s one question remaining . Does that mean that I can not install an i5 9 th gen or 8th gen 

Yes, unfortunately you cannot upgrade to 8th or 9th gen cpu.

Optiplex 3046 System chipset, Intel H110, which is 100 series.

Intel official statement:

9th and 8th Generation Intel® Core™ Desktop Processors use an LGA1151 socket. However, they require motherboards based on Intel® 300 Series Chipset. Some Intel® 300 Series Chipset based motherboards may need a BIOS update to support 9th Generation Intel® Core™ Desktop Processors.

These processors aren't backward compatible with motherboards based on Intel® 200 Series Chipset and Intel® 100 Series Chipset.

Similarly, 7th Generation Intel® Core™ Processors and 6th Generation Intel® Core™ Processors aren't compatible with the Intel® 300 Series Chipset motherboards.

bois

Can Dell Optipex 3046 Motherboard be applied with cpu intel core i5 8400?

@redxps630​ can i put intel 7th gen on my dell optiplex 3046? 

There is little gain upgrade from 6th to 7th gen.  Dell usually does not write bios update to support 7th gen cpu on 100 series chipset model such as 3046.