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A: none of the above. Get a zoom. What you loose in speed, you gain in versatility (and high ISO capabilities). My pick would be the old 28-105mm 3.5-4.5. A great lens even on the D750.
Yup. Even if i'm traveling I'll take a zoom and a prime. I use the Canon equivalent, the old Canon 28-105mm 3.5-4.5, which I'll use when I'm out specifically for photography. I'll stick the 40mm f2.8 (tiny) pancake on when I want something discreet or I'm out for the night
A: none of the above. Get a zoom. What you loose in speed, you gain in versatility (and high ISO capabilities). My pick would be the old 28-105mm 3.5-4.5. A great lens even on the D750.