Explanation:

Let’s look at the options and their fit for the sentence:

“If you don’t have a lump sum to invest in a fixed deposit, you can (1)______ opt for a longterm recurring deposit...”

- Option 1: sure

Not grammatically correct in this position. “You can sure opt” is not proper usage.

- Option 2: assured

Does not fit grammatically either. “You can assured opt” is not proper English.

- Option 3: surely

Means “certainly” or “definitely.” But in this context, the tone is about an alternative action, so it's a bit awkward in this formal instructional sentence.

- Option 4: ensure

“To ensure” would not work with “you can” (“you can ensure opt”), so this is not correct.

The most natural phrase is “you can opt for...” with no adverb at all. However, if adding an adverb, “surely” is possible but not the best. The most correct and idiomatic is actually “simply” (not listed). Since none is perfect, "surely" is the one that can fit but can sound awkward.

Given the choices, Option 3: surely is the closest to being appropriate, even though in very formal English, we would just say "you can opt..."

Option 3: surely (correct choice among given options)

Brief explanation:

- “Surely” works to mean “certainly” or for suggestion, making it the best fit.

- Other options are not grammatically correct here.

- The sentence tells the reader an alternative way to invest if they don’t have a lump sum.

- The passage is just saying you can, with confidence, choose a recurring deposit with small monthly amounts.