SC- pilgrimage

C it is
This is testing the gerund phrase form. Gerund phrases act as noun(either subject or object). Let's break down the sentence.

Making the pilgrimage to Mecca at least once in a lifetime, if they can afford it, is one of the five obligations of devout Muslims.

The bolded portion is a gerund phrase acting as a subject noun. Replace it by X and you will see what I mean:
X, if they can afford it, is one of the five obligations of devout Muslims.

Now, when speaking of "to afford", something is either affordable or someone can afford something. Since "if they can afford it is only a parenthetical element, it can be removed and the sentence must make sense.
subject: X(also a noun)
verb: is
object: one of the five obligations of devout Muslims

Hence, we are not saying that X is affordable but that X is one of the five obligations[...], what is between the bracket must end in the object form "affordable".
"if they can afford it" --> "they" properly refers to "devout Muslims" and "it" properly refers to X(Making the pilgrimage to Mecca at least once in a lifetime)
C is best

Good explanation Paul. I picked up B because i thought the pronoun 'it' in the option B was refering to the pilgrimage, and the verb form in B seemed to be right. After reading ur explanation, I am thinking again. Tell me something, in ur option E, do you feel the pronoun 'they' has a proper reference.

Thanks

Did you mean "it"? "it" has the proper referral going back to the pilgrimage. However, the problem is with the use of double conditional. "if it should be affordable". You either say "it should be affordable" in an affirmative way, or you say "if it is affordable" in a conditional way.

One question...pls clarify.

If muslims are 'obliged' to go to Mecca. Then 'they can afford it' or 'if affordable' can be wrong. 'when affordable' may be right. Pls explain - from a meaning point of view. Grammatically I agree that 'they can afford it' is well explained by Paul.

I am not sure whether the reasoning you supplied is correct or not. I think the way I will look at this question is that B, D, E can be eliminated based on wordiness and inappropriate usage of "when".

So the choice is between A and C. A is wrong because "if affordable" will modify "making the pilgrimage" and not to "pilgrimage". The idea the sentence tends to convey is that the "pilgrimage" should be described as affordable not "making"