CORRECT!

A glance at the approach plate for LDA RWY 6 approach indicates that CLAMM intersection is one of the initial approach fixes (IAF)s for the approach. It is also the final approach fix, indicated by the bold 'X' symbol appearing in the lower plan-view portion of the plate. An altitude restriction is associated with CLAMM intersection [ *4500 ].

The asterisk before the altitude refers the pilot to the note appearing immediately below [*4200 when glide slope not used]. When the glide slope is not used, the LDA 6 specifications apply, with a minimum descent altitude [MDA] of 1800 vs. an MDA of 1540 as would be the case if a glide slope is used [see the LDA/GS 6 minimums].

Now, referring back to the specific wording of the question, we need to determine the minimum altitude for crossing CLAMM intersection during the S LDA 6 approach. The answer, thus, is 4,200 MSL.

Answer a) is incorrect because the altitude indication '4182' appearing below CLAMM indicates the altitude the pilot would expect to be crossing CLAMM if following the glide slope. Answer b) is incorrect because, as noted on the chart, 2800 is the altitude associated with the next leg of the LDA/LOC approach [no glide slope], and only after crossing CLAMM should the aircraft descend to 2800 and maintain that altitude until crossing SKIRT outer marker.