This really sounds simple...simply find 15 caches with the word "challenge" in the title. Now here's the rub...no puzzle caches allowed, no locationless or lab caches, and only three caches from a series are acceptable. The words "challenged," "challenging," and "challenger" are not permitted. The caches you need lie few and far between, making this simple challenge not quite so simple...Good luck in your travels...
H69C----Pass Road challenge---------11-27-04 [trad]
H5B8----Hammond's Challenge-----12-15-04 [trad]
QENA----Austin Park's...Challenge---9-09-05 [trad]
TA97----Challenge 2-----------------------2-05-06 [trad]
P3AN----The Challenge...5 Bucks------2-11-06 [trad]
H4C9----Mesquite Challenge------------1-15-07 [trad]
1MCWB--The Challenge-------------------2-19-09 [trad]
14H10---...Crimson Kid Challenge-------8-09-07 [trad]
2PG9N---Keeping the challenge alive---3-21-11 [trad]
1W4XE---N.O.P.C....Challenge...-----------6-12-12 [trad]
4GZQA---Challenge Alley Clean-Up------9-07-13 [CITO]
5FV5P---Eagle Point Cache Challenge--11-15-14 [trad]
4ZNZA---Gono's Challenge #1-------------7-03-14 [trad]
4ZNZD---Gono's Challenge #2-------------7-03-14 [trad]
4ZN2J----Gono's Challenge #3-------------4-04-14 [trad]
6GA19---Challenge Alley WIG--------------4-24-16 [wherigo]