What does Jose Marti see as the biggest danger Spanish America faces
from the outside world?

A. colonization
B. the lure of wealth in other nations
C. war with their neighbors
D. a takeover by the U.S.

In general, "(d) a takeover by the US" is the most appropriate answer.

Generally, Jose Marti was a campaigner against American Manifest
Destiny. That is, he was against the idea that the United States was
destined to dominate the western hemisphere and the Americas. (This
fear was well justified, since in 1898 the Spanish-American war meant
the US took over most of Spain's former colonial island holdings,
including the Philippines).

See source: http://www.cubanow.net/global/loader.php?secc=6&cont=people/num19/02.htm
for more details on Jose's general legacy.

But specifically, from Jose himself, there is a very clear letter he
wrote to Manuel Mercado clarifying his mission in life:

"I am in daily danger of giving my life for my country and duty for I
understand that duty and have the courage to carry it out-the duty of
preventing the United States from spreading through the Antilles as
Cuba gains its independence, and from empowering with that additional
strength our lands of America. All I have done so far, and all I will
do, is for this purpose...The Cuban war, a reality of higher priority
than the vague and scattered desires of the Cuban and Spanish
annexationists, whose alliance with the Spanish government would only
give them the relative power, has come to America in time to prevent
Cuba's annexation to the United States, even against all those freely
used forces. "

You can obtain a translation of the complete source at:
http://www.cubaminrex.cu/josemarti/jose%20marti%20vers%20ingles/marti-political%20testament-ing.htm