Why do scientists need a standard system of measurement?
a.
to allow for consistency with measurement data
b.
in order to use tools marked in customary units
c.
so they can keep measurement data confidential
d.
to record measurements with easier abbreviations

The answer is so that everyone is measuring the same distance and counting that distance as the same everywhere. one example from real life why it is important to use standard units is the loss of the NASA mars orbiter. some people decided that the units were to be in metric and others thought that the units were in imperial (english) system. so the orbiter was lost.

let's eliminate some choices
A. to allow for consistientcy with measurement data:
not really, just make sure the instruments are accurate.

B. to use tools marked in customary units: this could be true because a custom is something that is accepted in a wide area and if it is standard, then it is customary so b is the answer

C. so they can keep measurement data secret: nope

D. to record with easier abbreviations: not really

the answer is B

None of the given choices is worth much.

The scientific community ... ALL of them ... need a standard system
of measurement so that one scientist can understand the results of
another scientist's work. 

Science is not a bunch of recluse hermits each working alone in their
dusty dungeons.  Science is a  community.  Each scientist publishes
a very detailed description of what he tried and what results he got. 
If other scientists read it and get ideas of how they could chase
answers to the same questions, they can pick up where the first
scientist left off, build on his work, or go off in slightly different
directions. 

None of that can happen unless everyone precisely understands
everyone else's measurements.