What is the correct order of the dynamic markings from softest to loudest?

a.
mp, p, mf, f

b.
p, mp, mf, f

c.
f, mf, mp, p

d.
mf, f, mp, p

The correct order of the dynamic markings from softest to loudest is: 
B. p, mp, mf, f

These are its letter meanings.
p - piano → This means "soft"
mp - mezzo-piano → This means "half soft"
mf - mezzo-forte → This means "half loud"
f - forte → This means "loud"

The overall ranking of dynamic markings are:

ppp - pianississimo. This means "very very soft"
pp - pianissimo. This means "very soft".
più p - più piano. This means "softer".
p
mp
mf
f
più f -  più forte. This means "louder".
ff - fortissimo. This means "very loud"
fff - fortississimo. This means "very very loud"

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