1. ostentatious : modest : :
a.
vigilant : lazy
b.
reticent : talkative
c.
visionary : blind
d.
ostensible : silent
e.
olfactory : loud
2. spectrum : range : :
a.
repartee : departure
b.
repertoire : music
c.
vigil : insomnia
d.
repose : bed
e.
surveillance : observation

1) Ostentatious : modest ⇒ B. Reticent : talkative
Ostentatious and modest are words whose definition are the exact opposite of one another. So, reticent and talkative fit the criteria of words that are antonyms.

2) Spectrum : range ⇒ E. Surveillance : observation
Spectrum is a condition that is not limited to a specific set of data or values.
Range is a condition that has limitations, from the smallest value to the largest value. 
Surveillance and observation fits the criteria wherein observation can be done within a surveillance just as range is part of a spectrum.

When you compare two things without using like or as thats what it means