lheiannie07 wrote:Captain Matthew Webb achieved fame in 1875 by being the first man to swim unaided across the English channel; eight years later, as he attempted to garner more acclaim, swimming across Niagara Falls was to cost him his life.

A)swimming across Niagara Falls was to cost him
B)swimming across the Niagara Falls were to cost
C)by swimming across the Niagara Falls he had cost
D)a swim across the Niagara Falls was costing him
E)during a swim across the Niagara Falls, came at a cost to

In B, were (plural) does not agree with swimming (singular).
Eliminate B.

The past perfect (had + VERBed) serves to express a past action completed BEFORE another past event.
C: eight years later...he had cost
Here, the usage of had cost (past perfect) implies that the action in red was completed BEFORE the moment in blue.
Not the intended sequence.
The intended sequence is that Webb lost his life not before 1883 but IN 1883 -- eight years after achieving fame in 1975.
Eliminate C.

D: eight years later...a swim was costing
Here, was costing (past progressive) cannot serve to express an action performed eight years later (in the future).
Eliminate D.

In E, came (verb) lacks a subject.
The result is an incomplete sentence.
Eliminate E.

The correct answer is A.

was to do X is equivalent to would do X.
OA: Eight years later, swimming across Niagara Falls was to cost him his life.
Conveyed meaning:
Eight years later, swimming across Niagara Falls WOULD COST him his life.