Step 1: Recall the usual crystal structure of each metal. Rather than starting with chromium, it helps to first place the other three metals, since their structures are very well known reference points in materials science. Step 2: Place the other metals first. Copper has a face centred cubic structure, which is exactly why it is so ductile and easy to draw into wire, magnesium crystallises as hexagonal close packed, which is part of why it is comparatively brittle at room temperature, and titanium is also hexagonal close packed at room temperature before it transforms to body centred cubic only above 882 degrees Celsius. Step 3: See what is left for chromium. None of copper, magnesium or titanium fit a room temperature body centred cubic description, which leaves chromium, and indeed chromium is one of the standard textbook examples of a metal with a body centred cubic lattice at ordinary temperatures. \[ \boxed{\text{Chromium}} \]