The grid in a high-vacuum triode is usually kept negatively charged with respect to the cathode so that the electrons may be
a. attracted to the cathode instead of the grid.
b. attracted to the anode instead of the grid.
c. accelerated toward the anode.
d. accelerated toward the cathode. (pf question)

The grid in a high-vacuum triode is usually kept negatively charged
with respect to the cathode so that the electrons may be attracted to
the anode instead of the grid (B).

I would bet [my own] money that they are not teaching you this material in
high school physics.  You will never ever have any use for this information,
unless maybe you're a contestant on Jeopardy some day, and the category
for Final Jeopardy is "Electronic Small-Signal Amplification Devices in the
First Half of the Twentieth Century".