Sentence Correction

Source: Grockit
The time when the apartment building in downtown Milwaukee started on fire has been determined by the fire department, but what is more problematic to establish are the source of the flame, the mechanism by which it ignited the building, and the extent of the damage.
a. has been determined by the fire department, but what is more problematic to establish are
b. has been determined by the fire department, but what is more problematic to establish is
c. have been determined by the fire department, but what is more problematic to establish is
d. have been determined by the fire department, but what is more problematic to establish are
e. are determined by the fire department, but that which is more problematic to establish is

I choose the answer as A because we for verb in end we have "the source of the flame,<modifier>, and the extent of the damage", which i thought should be plural. Am I right, if I am wrong why?

The Jock wrote:Source: Grockit
The time when the apartment building in downtown Milwaukee started on fire has been determined by the fire department, but what is more problematic to establish are the source of the flame, the mechanism by which it ignited the building, and the extent of the damage.
a. has been determined by the fire department, but what is more problematic to establish are
b. has been determined by the fire department, but what is more problematic to establish is
c. have been determined by the fire department, but what is more problematic to establish is
d. have been determined by the fire department, but what is more problematic to establish are
e. are determined by the fire department, but that which is more problematic to establish is

I choose the answer as A because we for verb in end we have "the source of the flame,<modifier>, and the extent of the damage", which i thought should be plural. Am I right, if I am wrong why?

Is the answer B?

The problem here is to determine the subject. The subject in the underlined part (after "BUT") is "what is more problematic".
Thus singular verb "is" is required.

Refer this thread:
https://www.manhattangmat.com/forums/pai ... t7994.html

The Jock wrote:Source: Grockit
The time when the apartment building in downtown Milwaukee started on fire has been determined by the fire department, but what is more problematic to establish are the source of the flame, the mechanism by which it ignited the building, and the extent of the damage.
a. has been determined by the fire department, but what is more problematic to establish are
b. has been determined by the fire department, but what is more problematic to establish is
c. have been determined by the fire department, but what is more problematic to establish is
d. have been determined by the fire department, but what is more problematic to establish are
e. are determined by the fire department, but that which is more problematic to establish is

I choose the answer as A because we for verb in end we have "the source of the flame,<modifier>, and the extent of the damage", which i thought should be plural. Am I right, if I am wrong why?

Two s-v issues.

Time [singular].Therefore has been.

Eliminate C,D,and E

now "What is more difficult to determine" is a noun clause .This entire clause is the subject of the following clause.Noun clauses are always singular.hence B

noun clause can start with "that"

For eg;

That the government is planning to reduce budget deficit is a good news for the markets.

The entire part in bold is noun clause and is the subject of the sentence

The verb is "is"

object is "news"

adjective modifying idea is "good"

prepositional phrase "for the markets"

object of the preposition " the markets [always a noun]"