General physics question

An observer O notes a particle moving to the right with velocity v. Another observer O ′ is moving with speed v relative to O along the direction of the motion of the particle. Which of the following is the value of the particle’s mass as measured by O and O ′?

a. O and O' both measure m0 

b. O and O' both measure m 

c. O measures m0 and O' measures m 

d. O measures m and O' measures m0