  We started a photometric survey using the WFC3/UVIS instrument onboard the
Hubble Space Telescope to search for multiple populations within Magellanic
Cloud star clusters at various ages. In this paper, we introduce this survey.
As first results of this programme, we also present multi-band photometric
observations of NGC 121 in different filters taken with the WFC3/UVIS and
ACS/WFC instruments. We analyze the colour-magnitude diagram (CMD) of NGC 121,
which is the only "classical" globular cluster within the Small Magellanic
Cloud. Thereby, we use the pseudo-colour
C_(F336W,F438W,F343N)=(F336W-F438W)-(F438W-F343N) to separate populations with
different C and N abundances. We show that the red giant branch splits up in
two distinct populations when using this colour combination. NGC 121 thus
appears to be similar to Galactic globular clusters in hosting multiple
populations. The fraction of enriched stars (N rich, C poor) in NGC 121 is
about 32% +/- 3%, which is lower than the median fraction found in Milky Way
globular clusters. The enriched population seems to be more centrally
concentrated compared to the primordial one. These results are consistent with
the recent results by Dalessandro et al. (2016). The morphology of the
Horizontal Branch in a CMD using the optical filters F555W and F814W is best
produced by a population with a spread in Helium of Delta(Y) =0.025+/-0.005.
