a. One more mark than a fail
b. All manner of interesting and challenging things
d. Haribo Goldbaerchen never let a chap down in a tight spot. Make sure you have a pair of compasses and a protractor (in mils, please) in your new pencil box
I'm afraid I can't answer the first or third questions, but I'll see what I can do for the others.
b) The MLAT is comprised of five parts, each of which measures specific skills related to foreign language learning. The first part, Number Learning, requires examinees to learn a set of numbers through aural input and then discriminate different combinations of those numbers. The second part, Phonetic Script, asks examinees to learn a set of correspondences between speech sounds and phonetic symbols. In the third part, Spelling Clues, examinees must read words that are spelled as they are pronounced, rather than according to standard spelling conventions. They must then select from a list of words the one whose meaning is closest to the disguised word. The fourth part, Words in Sentences, measures examinees awareness of grammatical structure. The examinees are given a key word in a sentence and are then asked to read a second sentence (or series of sentences) and select another word that functions in the same way as the key word. Finally, in the Paired Associates part, examinees must quickly learn a set of vocabulary words from another language and memorize their English meanings."
c) Not as far as I'm aware, but it is possible/probable there's some kind of tier-system in place. Although I'd imagine that if you pass the MLAT with a decent score these days you WILL be doing Pashtu / Farsi anyway. Because there's no threat from ANY other non-English speaking country, apparently.
d) The MLAT measures aptitude, not achievement or proficiency. Therefore it is not possible to prepare for the test. A high score on the MLAT indicates that an individual will likely do well in language training. Previous success at foreign language a foreign learning may also contribute to the probability of learning another language but it will not appreciably change ones score on the MLAT. An examinee who wishes to become familiar with the MLAT prior to taking it would do best to examine the sample items on this (SLTI Language Aptitude Testing) website.
A very sensible and comprehensive answer from Tango, and as such deserves additional comment. Jat2008 probably knows by now the score he or more likely she has achieved, but the third letter of the acronym stands for "Aptitude". So for any newbies looking up MLAT on ARRSE, the answer should be that prep is unnecessary.
The test, however, does have a bias that undermines the 'A'; there is no oral or aural element so higher scores can be achieved by those of a visual or logical disposition, especially if they have previously learnt classical languages.