A:Answer 2400mhz, and you'll need to stick to that when upgrading.
If you upgrade, get G.SKILL FORTIS Series 16GB (2 x 8GB) 288-Pin DDR4 SDRAM DDR4 2400 (PC4 19200) AMD X370 / B350 / A320 Memory (Desktop Memory) Model F4-2400C15D-16GFT. It's virtually guaranteed to work and is made specifically for Ryzen systems.